rename the next build back to nightly

Rename the build type, workflow, artifact and app id (com.subtracks.nightly)
from next to nightly, and update the signing docs to match.

.github/workflows/build-nightly.yml

@@ -0,0 +1,28 @@
+name: Build Nightly
+
+on:
+  push:
+    branches: [main]
+
+permissions:
+  contents: read
+
+jobs:
+  build:
+    runs-on: ubuntu-26.04
+    timeout-minutes: 60
+    permissions:
+      contents: read
+    steps:
+      - uses: actions/checkout@v7
+        with:
+          persist-credentials: false
+      - uses: ./.github/actions/build
+        with:
+          gradle-args: ":app:assembleDebug :app:assembleNightly --no-daemon"
+      - name: Checksums
+        run: sha256sum app/build/outputs/apk/debug/app-debug.apk app/build/outputs/apk/nightly/app-nightly.apk
+      - uses: actions/upload-artifact@v7
+        with:
+          name: subtracks-nightly
+          path: app/build/outputs/apk