[dark-mode] Phase 4.2: ThemeManager unit tests with JSDOM

Add 30 Mocha unit tests for ThemeManager covering all public methods and
edge cases using JSDOM for browser environment simulation.

New file (src/test/modules/themeManager.ts — 30 tests):
- init(): saved preference, system preference, default fallback, idempotent
- getCurrentTheme(): returns current theme, reads from DOM if not set
- setTheme(): applies theme and persists, emits themeChange event,
  rejects invalid values, handles localStorage quota errors
- applyTheme(): sets data-theme attribute, updates internal state
- getSystemPreference(): detects dark/light, caches result, handles errors
- hasSavedPreference(): checks localStorage key existence, handles errors
- getSavedPreference(): reads valid values, rejects invalid, handles errors
- emitThemeChange(): dispatches CustomEvent with theme detail
- Invalid value fallback: init() with invalid stored value falls back correctly

Dependencies:
- jsdom (devDependency) for browser environment simulation

Run with: npx ts-mocha src/test/modules/themeManager.ts --timeout 5000

Author: Hunta

.github/specs/dark-mode/documentation/progress-report.md

@@ -33,7 +33,7 @@
 | Phase | Description | Status |
 |-------|-------------|--------|
 | **4.1** | Code review & cleanup | ✅ Done |
-| **4.2** | Unit tests for ThemeManager | Not Started |
+| **4.2** | Unit tests for ThemeManager | ✅ Done |
 | **4.3** | Developer documentation | Not Started |
 | **4.4** | Update project documentation (README, DEVELOPMENT.md) | Not Started |
 | **5.1** | Prepare for merge (rebase, PR) | Not Started |