test: add comprehensive Vitest test suite (123 tests, 0 → full coverage of pure modules)

[Copilot]

The codebase had zero tests and no test infrastructure. This adds Vitest as the test framework and covers all testable pure modules — stores, utilities, hooks, and services.

Infrastructure

• Vitest 4.x + jsdom environment configured in vite.config.ts
• @testing-library/react + @testing-library/jest-dom for hook tests
• New scripts: test, test:watch, test:coverage
• src/test/setup.ts bootstraps jest-dom matchers globally

Test coverage added (11 files, 123 tests)

Module	Tests	Notes
lib/utils.ts	8	cn() — merging, deduplication, conditionals
stores/toast.ts	14	add/update/remove, all convenience methods, default messages
stores/adb.ts	19	metainfo parsing, device/display resolution, encoder list building, TinyH264 auto-selection
stores/file.ts	9	upload, getUploads, getApps with mocked service layer
services/.../storage.ts	17	local/session R-W-D, cookie > session > local priority, jsdom integration
services/.../credentials.ts	10	token generation, persistence, clear
services/.../http-client.ts	11	setBackendBaseUrl (protocol conversion, credential extraction, URL validation), setAuthToken
utils/file.js	6	getFileSizeInMb integer/fractional/large, getFileUrl
utils/rules.js	18	All validators — required, name, email, password — pass and fail branches
utils/mapClientToDevicePosition.js	7	All rotation values (0–3), clamping, rect offset
hooks/use-mobile.ts	4	Initial value, resize event, cleanup on unmount

Notable test patterns

• Zustand stores reset via setState({}) in beforeEach to prevent cross-test pollution
• Date.now mocked with an incrementing counter so rapid add() calls in the toast store produce unique IDs
• adbService and fileUploadService fully mocked to isolate store logic from HTTP