test: Add comprehensive unit tests

- IdleTimerTest.kt: 6 tests for timeout calculations
- ConfigValidationTest.kt: 16 tests for config validation
  - Homepage URL validation
  - Admin PIN validation (4-8 digits)
  - Whitelist parsing and normalization
- Updated IMPROVEMENTS.md with test coverage

Total unit tests: 29

IMPROVEMENTS.md

@@ -35,10 +35,23 @@ return whitelist.any { domain ->
 
 | Component | Tests | Status |
 |-----------|-------|--------|
-| Whitelist matching | 6 | ✅ |
-| Idle timer | TBD | ⏳ |
-| Admin PIN | TBD | ⏳ |
-| Boot receiver | TBD | ⏳ |
+| Whitelist matching | 7 | ✅ |
+| Idle timer | 6 | ✅ |
+| Config validation | 16 | ✅ |
+| Boot receiver | - | N/A (integration) |
+
+**Total Unit Tests: 29**
+
+### Test Files
+- `WhitelistTest.kt` — URL whitelist logic with bypass prevention
+- `IdleTimerTest.kt` — Timeout calculations and edge cases
+- `ConfigValidationTest.kt` — Homepage, PIN, and whitelist validation
+
+### Future Tests (Instrumented)
+- [ ] WebView URL blocking behavior
+- [ ] Admin PIN dialog flow
+- [ ] Lock Task Mode integration
+- [ ] Boot receiver launch
 
 ---