add a separate, editable up-next queue (#61)

Split playback into a shuffled context and a manual up-next list.
`up_next_entries` holds the same reference-plus-range rows as the
context but is never shuffled; `playback_cursor.upNextAnchor` pins the
block between a context track and its successor. `QueueRepository`
resolves a combined position by mapping through the context order and
splicing the block after the anchor, so the controller's window and
next/previous keep working.

PlaybackController gains addToQueue/playNext, consumes a queued track
when playback advances out of it, resumes the context when the block
empties, and clears the block on a new context. The queue view gets an
editable "Up next" section and a read-only context section, and the
context menu gains Play next / Add to queue for songs, albums and
playlists.

docs/architecture.md

@@ -84,11 +84,13 @@ The queue lives in SQL, not memory. `queue_entries` is an ordered list of *refer
 
 `QueueRepository` resolves a queue position to a song lazily: it walks the entries summing their lengths (a `COUNT(*)` per entry) and then fetches one row with `LIMIT 1 OFFSET`. `PlaybackController` keeps only the resolved entry lengths and the cursor in memory — never the songs — and hands the player a bounded window (currently `2 * 25 + 1` items) centred on the cursor. A player index maps back to a queue position as `windowStart + index`. The window is loaded once with `setMediaItems` and then shifted incrementally with `addMediaItems`/`removeMediaItem` as playback advances; a shift never replaces the playlist, so the current item and its prepared next source survive and transitions stay gapless. `hasNext`/`hasPrevious` are intentionally always true rather than derived from the SQL queue size: the previous/next buttons stay usable at both ends of the queue so repeat mode and restart-the-track keep working, and `next()`/`previous()` decide the actual end-of-queue action (advance, wrap when repeating, restart, or no-op). The snapshot is re-read whenever the window shifts and on skip, so a sync that shrinks the referenced album or playlist cannot strand the window on stale lengths; if the active source changes under a loaded queue, playback stops rather than resolving the old queue's stream URLs against the new server.
 
+Playback is a *context* plus a manual *up next* list. `queue_entries` is the context — the album, playlist or song you started, and the only thing shuffle permutes. `up_next_entries` is the manual queue: the same reference-plus-range rows, ordered, never shuffled. A `playback_cursor.upNextAnchor` (a flat index into the context) pins the block between a context track and its successor, and `QueueRepository` resolves a combined position by mapping it through the context order and splicing the block after the anchor, so the controller's window and next/previous logic keep working. Add to queue appends to the block and play next inserts at its front; both reuse the context edit machinery (range split, insert, compact). Advancing out of a block track removes it — consume as played, with no history — and a new context clears the block, so a manual edit never disturbs the shuffle permutation.
+
 Because the resolver reports *which entry* a position falls in, `PlaybackState` also carries the current queue context (`kind` and `refId`). That is what scopes the playing-track indicator in the album and playlist lists: a song that appears in both only lights up in the list the queue was actually started from.
 
 Stream URLs come from `SourceRepository.streamUri`, which asks the active `SubsonicSource` for a freshly salted `stream` URL; because the URL changes every request, artwork is attached to the media item by identity (the `CoverArtRef` cache key) rather than by URL. The active source carries the stream quality (a max bitrate plus an optional transcode format) for the current network: `NetworkMonitor` reports Wi-Fi vs mobile (metered) and `UserPreferences` stores a separate `StreamQuality` per mode, so `SourceRepository` rebuilds the source when either the mode or a quality changes. `PlaybackController` observes that effective quality and reloads the window around the current position, preserving play/pause and the playback position, so a Wi-Fi↔mobile transition re-issues the stream URLs with the new settings (a no-op when the two modes share a quality). Transcoded responses are chunked with no length and (because ffmpeg cannot seek back in a stream) no MP3 duration header, so Media3 would treat them as live and ignore seeks; the server's `estimateContentLength` is no better, since its estimate under-counts and cuts the track short. So a transcoded URL carries a declared upper-bound length in its fragment (never sent to the server), `KnownLengthDataSource` hands that length to Media3 to keep the stream seekable without truncation, and each transcoded item is clipped to the library duration so the timeline ends where the audio does and the next track still advances.
 
-The window is also the queue the media notification and Android Auto can browse — a deliberate trade-off, since the alternative is holding an entire large library in memory. The queue view resolves positions lazily too: it loads the queue around the cursor in chunks and extends by a chunk when you scroll near either end, so it never materializes the whole queue. Dragging uses `sh.calvin.reorderable`, which floats the lifted row and reorders the loaded list live (animating with `animateItem`), so editing is what a drag needs: the view reorders its loaded list while dragging and commits one move on drop. `queue_entries` is rewritten to edit: removing a position splits its entry into up to two ranges, and moving one turns the moved track into an explicit song entry inserted at the target index, so an edit costs a couple of rows rather than a copy of the queue. When the edit falls inside the player's window the controller mirrors it with `removeMediaItem`/`moveMediaItem` so playback is not interrupted; removing the playing track, or a move that crosses the window boundary, reloads the window around the cursor. The controller keeps the pre-edit entries and cursor, so the queue view can offer a one-step undo. Timed lyrics/gapless format preferences, scrobbling, downloads and adding to the queue from the library are not wired yet.
+The window is also the queue the media notification and Android Auto can browse — a deliberate trade-off, since the alternative is holding an entire large library in memory. The queue view resolves positions lazily too: it loads the queue around the cursor in chunks and extends by a chunk when you scroll near either end, so it never materializes the whole queue. Dragging uses `sh.calvin.reorderable`, which floats the lifted row and reorders the loaded list live (animating with `animateItem`), so editing is what a drag needs: the view reorders its loaded list while dragging and commits one move on drop. `queue_entries` is rewritten to edit: removing a position splits its entry into up to two ranges, and moving one turns the moved track into an explicit song entry inserted at the target index, so an edit costs a couple of rows rather than a copy of the queue. When the edit falls inside the player's window the controller mirrors it with `removeMediaItem`/`moveMediaItem` so playback is not interrupted; removing the playing track, or a move that crosses the window boundary, reloads the window around the cursor. The controller keeps the pre-edit entries and cursor, so the queue view can offer a one-step undo. The up-next block is fully editable in the same view (drag reorder and delete), while the context is read-only while shuffled. Timed lyrics/gapless format preferences, scrobbling and downloads are not wired yet.
 
 ## Data model
 

docs/implementation-plan.md

@@ -35,7 +35,8 @@ Not done yet in this slice (deliberately): cover-art tonal colour extraction, lo
 - (done) A SQL-backed queue of references (whole playlist / album / song, each with an optional ordinal range) resolved lazily, with only a bounded window in memory and in the player.
 - (done) Streaming bitrate and preferred format from settings (`maxBitRate`/`format`, so the server transcodes as configured).
 - (done) A queue view that lists the resolved queue, jumps to a track on tap, removes tracks and reorders them by dragging, with a one-step undo.
-- Adding to the queue from the library (play next / add to queue), and gapless format preferences.
+- (done) Adding to the queue from the library (play next / add to queue), backed by a separate, editable up-next list that plays between the current track and the rest of the context.
+- Gapless format preferences.
 - Scrobbling and "now playing" back to the server.
 - Optional download/caching.