docs: add mdBook docs-site, publish workflow, and release-notes tooling

Add a docs-site (mdBook) with a Gitea publish-docs workflow, a
release-notes generator script (release:notes) that turns a commit
range's TRACES into grouped notes, the background-audio feature spec,
and CLAUDE.md. Ignore docs-site build artifacts.

docs/ux-flows.md

@@ -690,24 +690,59 @@ flowchart TB
 └─────────────────────────────────────────┘
 ```
 
-### 9.2 Video Playback in Background
+### 9.2 Video Playback in Background (Android — PiP & Background Audio)
+
+Leaving the app while a **local video** is playing does not simply pause it.
+What happens depends on which background behaviour is active. The two are
+**mutually exclusive**, and both apply **only to locally-rendering video** —
+audio-only playback, library/menu browsing, and remote/cast sessions never
+trigger PiP (see decision gate below).
 
 ```mermaid
 flowchart TB
-    VideoPlaying[Video Playing] --> Background{User Action}
+    Leave[User leaves app<br/>Home / gesture / screen lock] --> Gate{Local video surface<br/>actively rendering?<br/>canEnterPip}
 
-    Background -->|Home Button| AutoPause[Automatically Pause]
-    Background -->|Screen Lock| AutoPause
+    Gate -->|No — audio, browsing,<br/>or remote/cast| Normal[App backgrounds normally<br/>audio, if any, continues via<br/>media notification &#40;§9.1&#41;]
 
-    AutoPause --> SaveProgress[Save Progress]
-    SaveProgress --> ShowNotification[Show Paused Notification:<br/>"Tap to Resume"]
+    Gate -->|Yes| Mode{Background mode armed?}
 
-    ShowNotification --> UserReturn{User Returns?}
+    Mode -->|Background-audio toggle ON<br/>UR-040| Handoff[Hand off to native audio service<br/>WebView &lt;video&gt; torn down,<br/>video decode stops, audio continues]
+    Mode -->|Default<br/>UR-041| PiP[Auto-enter Picture-in-Picture<br/>on onUserLeaveHint]
 
-    UserReturn -->|Tap Notification| ResumeVideo[Open App to Video Player]
-    UserReturn -->|Later| KeepPaused[Video Remains Paused]
+    PiP --> PiPWindow[Floating PiP window:<br/>- Video keeps rendering into surface<br/>- WebView hidden<br/>- Play/Pause RemoteAction<br/>  &#40;reflects live player state&#41;]
 
-    ResumeVideo --> AskResume[Resume from Saved Position]
+    PiPWindow --> PiPReturn{User action}
+    PiPReturn -->|Tap window| Restore[Return to full player<br/>WebView restored, surface re-fit]
+    PiPReturn -->|Close window| Stop[Playback stops]
+
+    Handoff --> Foreground[On return to foreground:<br/>resume WebView video at position]
+```
+
+**Key rules:**
+
+- **Video-only gate.** Auto-PiP is guarded by the native `canEnterPip` check
+  (local video surface actively rendering). Audio playback and menu/library
+  browsing background normally; remote/cast sessions render nothing locally, so
+  a PiP window would be an empty box and is refused. *(UR-041, IR-026)*
+- **Only one background behaviour at a time.** The background-audio toggle
+  (UR-040) disarms auto-PiP while it is on, so a video is either handed to the
+  audio service *or* floated in PiP, never both.
+- **PiP controls track the player.** The play/pause RemoteAction in the PiP
+  window reflects the live player state and updates on every playback-state
+  change, not only when the button is pressed. *(DR-053)*
+- **Non-disruptive transition.** ExoPlayer keeps rendering into the same
+  surface across enter/exit, so entering or leaving PiP never interrupts the
+  video; on exit the surface is re-fit to full-screen bounds. *(DR-053)*
+
+**PiP window (Android):**
+```
+        ┌───────────────────┐
+        │                   │
+        │   ▶ video frame   │
+        │                   │
+        │       [⏸]         │  ← play/pause RemoteAction
+        └───────────────────┘
+   sized to the video's aspect ratio
 ```
 
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