Stabilisation quick fixes: orderly close, visible version and update band, chat pop-ups on, Windows shaders and hwdec, render logs

- Close: hide the window, dispose every player, thumbnail job and the
  sound engine, leave, then close normally; Linux runner _exit (the
  NVIDIA EGL exit handler crashed 10/10 closes, 0/41 now); Windows tears
  down inside the message loop, then TerminateProcess; watchdogs.
- Version under the Settings tab; an update band instead of a dot,
  never over a video; checks at start and every 4 hours.
- Chat pop-ups default to All again.
- Shader read-back compared as a list (old mpv joins with ','), so
  Windows stops turning shaders off; Windows hwdec auto-copy.
- Render diagnostics (`mpv: render` lines); RAM-only demuxer cache pin
  silences the file-cache warning; a volume readout test.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

docs/DEVELOPMENT.md

@@ -176,6 +176,19 @@ headers), `mpv:` (`cannot open the media` and ffmpeg's lines), `yt-dlp:`
 the headers sent). A viewer that got no bytes while the source was
 slow to come up shows mpv's error with "no HTTP error logged".
 
+For "the video is slow / shaders do nothing", grep `mpv: render` (what
+each line holds: "Render mode" in [ARCHITECTURE.md](ARCHITECTURE.md)).
+`render: gpu` with a real `GL_RENDERER` (not `llvmpipe` / `softpipe`) is
+the good case; `render: softwareGl` is OpenGL on the CPU (Mesa's software
+rasterizer: missing or blocked GPU drivers, `LIBGL_ALWAYS_SOFTWARE=1`);
+`render: software` with no GL lines is media_kit's own fallback (it found
+no usable EGL context: check `render env:` for an X11 session or a
+`GDK_BACKEND` / `FLUTTER_LINUX_RENDERER` override). Its reason is only on
+stderr: have the user start the app from a terminal
+(`watch-together 2>&1 | grep VideoOutput`). `hwdec-current=no` on the
+first-frame line means software decoding (no hwdec for that codec, or it
+is off), not software rendering.
+
 ## Gotchas
 
 - Relic gives Range as a typed `RangeHeader`; convert from unit + first range.