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-lock.json

@@ -108,6 +108,7 @@
         "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",
@@ -12357,6 +12358,25 @@
       "resolved": "https://registry.npmjs.org/tslib/-/tslib-2.8.1.tgz",
       "integrity": "sha512-oJFu94HQb+KVduSUQL7wnpmqnfmLsOA/nAh6b6EH0wCEoK0/mPeXU6c3wKDV83MkOuHPRHtSXKKU99IBazS/2w=="
     },
+    "node_modules/tsx": {
+      "version": "4.22.4",
+      "resolved": "https://registry.npmjs.org/tsx/-/tsx-4.22.4.tgz",
+      "integrity": "sha512-X8EX+XV4QR5xCsrgxaED954zTDfY8KqlDtskKEL0cHhyS/P8b4IFOvGDQpsC9Q1XnLq915wEfwwY/zzskCtmhg==",
+      "dev": true,
+      "license": "MIT",
+      "dependencies": {
+        "esbuild": "~0.28.0"
+      },
+      "bin": {
+        "tsx": "dist/cli.mjs"
+      },
+      "engines": {
+        "node": ">=18.0.0"
+      },
+      "optionalDependencies": {
+        "fsevents": "~2.3.3"
+      }
+    },
     "node_modules/type-check": {
       "version": "0.4.0",
       "resolved": "https://registry.npmjs.org/type-check/-/type-check-0.4.0.tgz",