fix(ui): declare Material system tokens and migrate dead --mdc overrides

The theme is built with the legacy mat.define-theme config, whose component
mixins never declare the --mat-sys-* system variables, so every app style
that read one resolved to nothing. Declare them with mat.system-level-* for
the light (html) and dark (.dark-theme) contexts.

Material 22 reads only --mat-* component tokens, so the remaining --mdc-*
overrides compiled but did nothing. Move them to the mat.*-overrides()
mixins (which reject unknown names at build time) and add a CI guard,
styles:material-tokens:validate, that rejects retired --mdc-* names.

Where a now-resolving token changed a shipped surface for the worse, keep
the shipped look:
- form fields keep the theme's outline roles (the hairline app token gave
  inputs a 1.4:1 boundary) and default text sizes (a 13px field floated a
  10px label);
- M3U and collection live views and the radio hero sit on --app-content-bg
  like the Xtream/Stalker live views, instead of the M3 neutral surface;
- the rail brand stays untiled.

Adds the global .app-destructive-button class (adopted by the follow-up
confirmation change) and the theme-tokens Electron E2E, and updates the
token policy in the UI guidelines and the theme skill.

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

libs/workspace/shell/feature/src/lib/workspace-shell/components/workspace-shell-rail/workspace-shell-rail.component.scss

@@ -100,8 +100,6 @@
 
 .brand {
     margin-bottom: 10px;
-    background: var(--mat-sys-primary-container);
-    color: var(--mat-sys-on-primary-container);
     width: 40px;
     height: 40px;
     border-radius: 12px;