agent: lumotia — Phase A.5 vitest scaffold + localStorageMigration unit tests

First frontend unit test framework on Lumotia. Pinned exact versions for
supply-chain hygiene (matches the rust-toolchain.toml discipline from the
27661c8 hygiene pass and the npm audit signatures pre-flight from e4d56b8):

  - vitest 4.1.6 (compatible with vite 6, supports vite 6/7/8)
  - jsdom 29.1.1

Installed with `npm install --save-dev --save-exact --ignore-scripts` per
the install discipline documented in the README — the --ignore-scripts
flag blocks the postinstall vector that npm worms (Shai-Hulud,
mini-Shai-Hulud) rely on.

vite.config.js:
  - Switched defineConfig import to vitest/config (superset of vite/config;
    production builds ignore the `test` key).
  - test.environment = "jsdom" so storage-shim tests drive real browser APIs.
  - test.include scoped to src/**/*.{test,spec}.{ts,js} — colocated with
    source, mirrors the Rust #[cfg(test)] sibling pattern.
  - test.exclude blocks src-tauri/ (owned by cargo test).
  - restoreMocks + clearMocks + unstubAllGlobals on so module-level state
    can't leak between tests.

src/lib/utils/localStorageMigration.test.ts — 12 tests:
  migrateLocalStorageKey:
    - copies value + removes old when only old exists
    - removes old + keeps new when both exist (lumotia is authoritative)
    - no-op when only new exists
    - no-op when neither exists
    - idempotent (second call after first migrates nothing)
    - preserves the value's exact bytes (no JSON round-trip)
    - preserves empty-string values (distinct from null)
    - survives DOMException / quota errors without re-raising
    - no-op when localStorage is undefined (SSR-safe)
  migrateLocalStorageKeys:
    - processes pairs in order
    - per-pair failure does not strand remaining pairs (resilience)
    - empty pairs list is a clean no-op

package.json:
  - "test": "vitest run" (one-shot, CI-friendly)
  - "test:watch": "vitest" (dev loop)

README: documents `npm run test` alongside `cargo test --workspace` and
`npm run check` in the Testing section.

Verification:
- npm run test: 12/12 pass
- npm run check: 0 errors, 0 warnings (the new .ts test type-checks clean
  against jsconfig.json's strict typescript settings)

package.json

@@ -11,6 +11,8 @@
     "preview": "vite preview",
     "check": "svelte-kit sync && svelte-check --tsconfig ./jsconfig.json",
     "check:watch": "svelte-kit sync && svelte-check --tsconfig ./jsconfig.json --watch",
+    "test": "vitest run",
+    "test:watch": "vitest",
     "tauri": "tauri"
   },
   "license": "MIT",
@@ -31,10 +33,12 @@
     "@sveltejs/vite-plugin-svelte": "^5.0.0",
     "@tailwindcss/vite": "^4.2.1",
     "@tauri-apps/cli": "^2",
+    "jsdom": "29.1.1",
     "svelte": "^5.0.0",
     "svelte-check": "^4.0.0",
     "tailwindcss": "^4.2.1",
     "typescript": "~5.6.2",
-    "vite": "^6.4.2"
+    "vite": "^6.4.2",
+    "vitest": "4.1.6"
   }
 }