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>

package.json

@@ -16,6 +16,7 @@
     "check:prettier": "prettier --check .",
     "fix:prettier": "prettier --write .",
     "typecheck": "tsc --noEmit",
+    "test": "node --import tsx --test $(find src -name '*.test.ts')",
     "prepare": "husky",
     "commit": "git-cz",
     "postinstall": "node scripts/patch-folds.mjs",
@@ -132,6 +133,7 @@
     "husky": "9.1.7",
     "lint-staged": "17.0.5",
     "prettier": "3.8.3",
+    "tsx": "4.22.4",
     "typescript": "6.0.3",
     "vite": "8.0.14",
     "vite-plugin-pwa": "1.3.0",