refactor(styles): move timetable tokens from .scss to .css for test-time loadability

Per Phase A finding A2 — `_timetable.scss` was functionally pure CSS:
only :root custom properties + @keyframes + one .tt-cascade-pulse class.
The only SCSS-specific syntax was `// line comments`. Zero $vars, @use,
@mixin, @function, nesting, or color functions.

Why move to .css: Vitest+jsdom can `import '@/styles/tokens/_timetable.css'`
directly so getComputedStyle() resolves var(--tt-…) in component tests
(needed for the upcoming PerformanceBlock visual-state assertions). SCSS
imports require Vite's SCSS plugin, which the vitest.config.ts intentionally
skips for unit-test speed.

Changes:
- `_timetable.scss` → `_timetable.css` (line comments converted to /* */
  block comments; everything else byte-identical)
- `assets/styles/styles.scss`: switch from `@use "@/styles/tokens/timetable"`
  to `@import "@/styles/tokens/_timetable.css"`
- Production `npm run build` passes (16s, no asset warnings)

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

apps/app/src/assets/styles/styles.scss

@@ -1,4 +1,5 @@
 // Write your overrides
 
 // RFC-TIMETABLE v0.2 D21 — status palette + geometry custom properties.
-@use "@/styles/tokens/timetable";
+// Plain CSS so jsdom/vitest can also load it via `import '@/styles/tokens/_timetable.css'`.
+@import "@/styles/tokens/_timetable.css";