chore: drop legacy TUI/dead code

- Delete audit/internal/app/panel.go (388 lines, zero callers — TUI panel remnant)
- Delete RenderGPULiveChart() from platform/gpu_metrics.go (~155 lines, never called)
- Move formatSATDetail/cleanSummaryKey helpers to app.go (still used)
- Update motd: replace bee-tui with Web UI hint
- Update journald.conf.d comment: remove bee-tui reference

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

iso/overlay/etc/motd

@@ -12,6 +12,6 @@
   Export dir:    /appdata/bee/export
   Self-check:    /appdata/bee/export/runtime-health.json
 
-  Open TUI:      bee-tui
+  Web UI:        http://<ip>/
 
   SSH access:    key auth (developers) or bee/eeb (password fallback)

iso/overlay/etc/systemd/journald.conf.d/bee.conf

@@ -1,4 +1,4 @@
 [Journal]
-# Do not forward service logs to the console — bee-tui runs on tty1
-# and log spam makes the screen unusable on physical monitors.
+# Do not forward service logs to the console — prevents log spam on
+# physical monitors and the local openbox desktop.
 ForwardToConsole=no