Add frontend unit tests with Vitest + jsdom

- Create tests/frontend/utils/format.test.js with 24 tests for formatting utilities
- Create tests/frontend/ui/downloads.test.js with 10 tests for DOM rendering functions
- Update vitest.config.js to support jsdom environment for frontend tests
- All 34 tests pass and cover edge cases (null, zero, large numbers, DOM structure)

vitest.config.js

@@ -3,8 +3,6 @@ import { defineConfig } from 'vitest/config';
 
 export default defineConfig({
   test: {
-    // Node environment for all tests (server-side CJS modules, no browser APIs needed)
-    environment: 'node',
     // Global test helpers (describe, it, expect, vi) without per-file imports
     globals: true,
     // Run each test file in an isolated module registry so module-level state
@@ -12,6 +10,14 @@ export default defineConfig({
     isolate: true,
     // Give each file its own data directory so tokenStore file I/O doesn't collide
     setupFiles: ['./tests/setup.js'],
+    // Environment configuration based on test type
+    environmentMatchGlobs: [
+      // Server tests use node environment (must come first - more specific)
+      ['tests/unit/**/*.js', 'node'],
+      ['tests/integration/**/*.js', 'node'],
+      // Frontend tests need jsdom for DOM APIs (broader pattern comes last)
+      ['tests/frontend/**/*.js', 'jsdom']
+    ],
     // Coverage via V8 (built into Node — no babel transform needed)
     coverage: {
       provider: 'v8',