Port the player to Raspberry Pi OS Trixie 64-bit (Linux-only branch)

Replaces the Windows port with a Raspberry Pi / Linux implementation on
Raspberry Pi OS "Trixie" (Debian 13, aarch64, Wayland/labwc). The Windows
code is removed here but preserved on the Windows-Player branch.

Entry point
-----------
linux/run_linux.py replaces windows/run_win.py. src/main.py stays
platform-neutral; all Pi-specific behaviour is injected from linux/.

Five bugs that prevented the port (all measured on real hardware)
----------------------------------------------------------------
1. Kivy's PyPI wheel bundles an SDL2 built WITHOUT the wayland driver, so
   no window could be created (Trixie has no X server). linux/fix_kivy_sdl2.sh
   symlinks the system SDL2 over the bundled filename.
2. SDL2 requires WAYLAND_DISPLAY to be *set* - the socket alone is not enough,
   unlike wlopm. This broke every systemd/cron/autostart launch.
   linux_display.ensure_session_environment() detects and exports it.
3. Kivy's Clock resolves callbacks via func.__name__; a patch assigned under a
   different name crashed the player ~20s after a successful start.
4. The inherited signal_screen_activity() shelled out to tvservice, xdotool and
   ydotool - none exist on Trixie - and mis-escaped 'wlopm --on \*', so the
   display blanked after 10 minutes.
5. The launchers ran src/main.py directly, bypassing every platform patch and
   resolving the data directory one level too high.

Web links
---------
- --ozone-platform-hint=auto does NOT fall back to Wayland on Chromium 152; it
  aborts. The platform is now chosen explicitly.
- The keyring password prompt is suppressed via the ENVIRONMENT, not the flags:
  launch_env() strips DBUS_SESSION_BUS_ADDRESS for the child so Chromium cannot
  reach gnome-keyring-daemon.
- Teardown kills the whole process group (needs start_new_session=True);
  previously it silently fell back to terminate() and orphaned children.

Video normalisation
-------------------
A 4K video cannot play on a Pi 4: ffpyplayer decodes in software, measured at
0.90x realtime (1080p is 3.03x). Oversized media is downscaled to 1920x1080 at
sync time using the hardware h264_v4l2m2m encoder (~31s for an 18s clip),
triggered by resolution only so already-playable files are untouched.

src/media_state.py owns the shared on-disk contract: a .kiwy-converting marker
makes the player skip the item while it is being rebuilt, then the converted
file is played instead. If nothing is playable at all (a single-item playlist
whose only video is converting), the player loops the intro video rather than
leaving a blank screen.

Also fixed
----------
- network_monitor: replaced netsh/ifconfig/dhclient with nmcli (Trixie uses
  NetworkManager; ifconfig and dhclient are not even installed).
- Removed the Windows-only focus keeper/guardian from main.py.
- main.py: duplicate SDL_AUDIODRIVER setdefault (a silent no-op); Settings
  "Test connection" now uses tempfile.gettempdir().
- config/app_config.json: credentials blanked so a fresh clone runs the
  first-run setup flow.

Verification
------------
linux/test_media_state.py 18/18, test_linux_patches.py 21/21,
test_linux_browser_flags.py 27/27. Verified live against a real DigiServer:
image -> weblink -> image -> video with correct durations, zero leaked Chromium
processes, and no throttling over a 10 minute monitored run.

start.sh

@@ -241,10 +241,17 @@ while true; do
     if [ -f "$SCRIPT_DIR/.venv/bin/activate" ]; then
         source "$SCRIPT_DIR/.venv/bin/activate"
     fi
-    
-    # Start the player
-    cd "$SCRIPT_DIR/src"
-    python3 main.py &
+
+    # Start the player.
+    #
+    # NOTE: the entry point is linux/run_linux.py, NOT src/main.py. Running
+    # main.py directly skips the Raspberry Pi platform patches (Wayland session
+    # environment, display keep-awake, Chromium kiosk adapter), which is why
+    # the player used to blank after 10 minutes and fail to show web links.
+    # Run from the project root so the data directory is the repo root and the
+    # relative config/media/playlists paths resolve as the app expects.
+    cd "$SCRIPT_DIR"
+    python3 linux/run_linux.py &
     PLAYER_PID=$!
     
     log_message "Player PID: $PLAYER_PID"