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/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.