fix: address code review findings (critical + important + minor)

Critical fixes:
- C1: Fix GDI handle leak in IconGenerator (lazy singletons + DestroyIcon)
- C2: Add appsettings.json to .gitignore, provide template
- C3: Properly dispose IHost on exit

Important fixes:
- I1: Add SpamGuardOptionsValidator for startup config validation
- I3: Use UID range queries instead of SearchQuery.All
- I4: Only scan recent Sent messages after initial full scan
- I5: Atomic file writes (write-to-temp, then rename)
- I6: Set 30s HTTP timeout and base address on HttpClient
- I7: Remove duplicate AddSingleton<EmailClassifier> (DI conflict)
- I8: Better HTML stripping (script/style removal, entity decoding)

Minor fixes:
- M1: Delete placeholder UnitTest1.cs
- M2: Wire MaxActivityLogEntries config to ActivityLog constructor
- M4: Log warnings instead of bare catch blocks
- M5: Dispose JsonDocument instances
- M8: Use AppContext.BaseDirectory for appsettings.json path
- M9: Truncate NotifyIcon.Text to 127 chars

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

.gitignore

@@ -482,3 +482,6 @@ $RECYCLE.BIN/
 
 # Vim temporary swap files
 *.swp
+
+# User configuration with secrets
+src/SpamGuard/appsettings.json