Remove the HA Dashboard tab and the Qt WebEngine dependency (D40)

DSH-1 ended with a negative result: the Frame's system WebEngine can't be
loaded, and bundling our own failed (no sandbox, ICU resources, Vulkan
abort, empty page). Reverts the tab (60deb43), the container packages
(0676998) and the bundler's WebEngine support (b823ff3, e2cfa8e), and
updates the docs.

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

docs/STEAM-FRAME.md

@@ -70,7 +70,8 @@ Without the deploy script: copy the `.tar.gz`, unpack it on the Frame, run `./in
 Checked 2026-10-05; details per requirement in [REQUIREMENTS.md](REQUIREMENTS.md), Appendix C.
 
 - **System Qt 6.8.0** under `/usr/lib` includes, besides the base modules: DBus, Network,
-  Multimedia (FFmpeg plugin), WebSockets, WebEngine (Core, Quick), WebView, TextToSpeech,
+  Multimedia (FFmpeg plugin), WebSockets, WebEngine (Core, Quick; not loadable, see below),
+  WebView, TextToSpeech,
   Positioning, Sensors, Svg, VirtualKeyboard. TLS plugins: `libqopensslbackend.so`,
   `libqcertonlybackend.so`.
 - It apparently comes with the KDE Plasma desktop mode (plugins for plasma, kwin, discover).
@@ -84,6 +85,13 @@ Checked 2026-10-05; details per requirement in [REQUIREMENTS.md](REQUIREMENTS.md
   depend on it. A program linking WebEngine fails to start ("libavcodec.so.60: cannot open
   shared object file"). Other Qt modules are 6.8.0 (`pacman -Q | grep qt6-`). Qt
   Multimedia's FFmpeg plugin resolves all its libraries (`ldd`, no "not found").
+- **Bundling our own Qt WebEngine failed** (experiment, 2026-10-05; Debian's Qt 6.8.2 incl.
+  WebEngine with its own FFmpeg, 141 MB package): the helper process (`QtWebEngineProcess`,
+  needs the executable bit) only starts without Chromium's sandbox (`--no-sandbox`); without
+  `QTWEBENGINE_RESOURCES_PATH` its child processes find no ICU data ("Invalid file descriptor
+  to ICU data"); with Vulkan the app aborts ("free(): invalid pointer"); with OpenGL the
+  dashboard tab stays empty; in a desktop window the page stays white and Chromium's
+  remote-debugging port never opens. FrameAssistant uses no web engine (D40).
 - **Qt add-on modules must come from the system Qt.** Shipping one fails: they use Qt's private
   API, and QtCore aborts on any version mismatch ("Cannot mix incompatible Qt library").
 - **Not present:** any MQTT library (no `libmosquitto`, no Qt MQTT). FrameAssistant ships