test: add unit-test harness (tsx + node:test) + first suite for utils/common

Addresses the "no automated test suite" gap. Chose Node's built-in test runner
via tsx rather than vitest: the project is on Vite 8.0.14, ahead of vitest's
supported Vite range, so vitest would fight peer deps. tsx is build-independent.

- `npm test` → `node --import tsx --test $(find src -name '*.test.ts')` (works on
  Node 20 local + 24 CI without relying on --test glob support).
- src/app/utils/common.test.ts: 15 tests covering the pure helpers (bytesToSize,
  time formatters, binarySearch, parseGeoUri, slash trimmers, nameInitials,
  randomStr, suffixRename, splitWithSpace, promise-settled helpers, etc.) —
  asserts actual behavior, traced from source.
- common.ts: folds import made `import type` (it's types only) so the module is
  pure and testable without loading folds/CSS.
- tsconfig excludes *.test.ts (tsx transpiles tests; eslint isn't type-aware so
  it still lints them); added an informational CI "Unit tests" step (promote to a
  hard gate by dropping continue-on-error).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

.gitea/workflows/ci.yml

@@ -48,6 +48,13 @@ jobs:
           VITE_APP_VERSION: ${{ github.sha }}
 
       # ── Quality checks (informational — pre-existing issues exist) ───────
+      # Unit tests run on Node's built-in runner via tsx (no vitest — Vite 8 is
+      # ahead of vitest's supported range). Informational for now to match the
+      # section's convention; promote to a hard gate by dropping continue-on-error.
+      - name: Unit tests
+        run: npm test
+        continue-on-error: true
+
       - name: TypeScript
         run: npm run typecheck
         continue-on-error: true