refactor: Convert history state to Svelte 5 runes + add svelte-check

Migrate the Timeline/Saved history state module to idiomatic Svelte 5
runes and wire up type-checking.

- Rename history.ts -> historyState.svelte.ts and replace the Svelte
  stores with $state-backed reactive values, removing the get() calls
  (the only remaining one reads the external inputStateStore). A small
  localStorage-backed `persisted()` helper keeps the same keys, so user
  data is preserved. Consumers read via the reactive `historyState`
  getter object instead of store auto-subscriptions.
- The gist loader now replaces the in-memory revisions in one call
  (setLoaderEntries) instead of appending, so loading a new gist no
  longer accumulates stale revisions.
- Add svelte-check: new `check` script, a "Type check" step in the
  unit-tests CI workflow, and skipLibCheck so the check passes on the
  dependency .d.ts files. svelte-check reports 0 errors / 0 warnings.
- Add accessible labels to the History toggle and the per-item
  Restore/Delete buttons (a11y + testability).
- Unskip tests/history.spec.ts and rewrite it against the new UI:
  load-from-localStorage + restore, tab-highlight follows the mode,
  save/dedupe, auto-vs-manual isolation, and delete/clear. All pass.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

tsconfig.json

@@ -12,6 +12,7 @@
     "resolveJsonModule": true,
     "allowSyntheticDefaultImports": true,
     "strictNullChecks": true,
+    "skipLibCheck": true,
     "types": ["vitest/importMeta", "@playwright/test"]
   },
   "extends": "./.svelte-kit/tsconfig.json"