Cleanup repo + harden offline install and watchdog

Repo hygiene:
- Remove dead scripts (.wait-for-display, .display-keepalive, .keep-screen-alive)
- Remove stale backup/scratch (install.sh.bak, New.txt, stale .pyc)
- Stop tracking machine-specific/generated state (config/app_config.json,
  playlists/server_playlist.json, src/player_auth.json, generated launchers)
- Prune duplicate wheel versions (one per package)
- Reorganize working_files -> dev/, dev/tests/, documentation/dev-notes/, scripts/
- Extend .gitignore; fix no-op rules in repo/.gitignore

Offline install (install.sh):
- Non-fatal system-dep step; per-package apt retry; .deb bundle install
- Skip pip self-upgrade offline; guard online fallbacks behind connectivity probe
- Dependency-free fetch_url (curl/wget/urllib); --no-reboot flag
- Fix corrupted literal backslash-bang escapes (shebangs, test -f, banner)
- Idempotent cron install (no more stacked @reboot lines)

Autostart / watchdog:
- start.sh: single-instance flock lock (.player_watchdog.lock) acquired only
  after a display is confirmed, preventing parallel watchdogs
- Set XDG_RUNTIME_DIR when missing; exit early if headless
- run_player.sh: activate .venv; stop_player.sh: clear lock file

documentation/dev-notes/OFFLINE_QUICKSTART.md

@@ -0,0 +1,43 @@
+# Offline Installation Quick Start
+
+## For Connected System (Preparation)
+
+```bash
+# 1. Download Python packages (required)
+bash scripts/download_offline_packages.sh
+
+# 2. Download system .deb packages (optional, for fully offline)
+bash scripts/download_deb_packages.sh
+```
+
+## For Offline System (Installation)
+
+```bash
+# The installer auto-detects offline packages
+bash install.sh
+
+# Or explicitly use offline mode
+bash install.sh --offline
+```
+
+## What's Included
+
+### Python Packages
+✅ Kivy 2.3.1  
+✅ Requests 2.34.2  
+✅ Bcrypt 4.2.1  
+✅ Aiohttp 3.11.16 (async HTTP)  
+✅ ffpyplayer 4.5.3 (video)  
+✅ evdev 1.9.1 (card reader)  
+✅ All dependencies
+
+### System Packages
+📋 See `repo/system-packages/apt-packages.txt`
+
+## File Size
+- Python wheels: ~50 MB
+- System packages: ~200-500 MB (if .deb downloaded)
+
+## See Also
+- **OFFLINE_INSTALLATION.md** - Complete guide
+- **repo/README.md** - Repository structure