chore: clean up repo organization

- Standardize on bun: remove package-lock.json, add packageManager field,
  gitignore non-bun lockfiles, fix stray npm install in android:build:clean
- Remove stale build logs and empty dirs (src-tauri/plugins, docs/tickets)
- Move android-dev.sh into scripts/
- Consolidate root docs into docs/ (docker/builder under docs/build/);
  move the architecture overview to docs/architecture/README.md
- Extract Requirements Specification from README into docs/requirements.md
  and slim README down to a project intro + docs index
- Fix internal references to the moved files

src-tauri/src/player/session.rs

@@ -5,7 +5,7 @@
  * beyond individual playback states. Enables persistent UI (miniplayer) and
  * proper transitions between content types.
  *
- * See software-architecture.md Section 2.1 for state machine diagram.
+ * See docs/architecture/01-rust-backend.md for the state machine diagram.
  */
 
 use log::info;