chore: remove unused code and suppress noisy Claude Code logs

- Delete unused files: retry.js, app-init.js, model-manager.js
- Remove duplicate error helpers from helpers.js (exist in errors.js)
- Remove unused exports from signature-cache.js, logger.js
- Remove unused frontend code: ErrorHandler methods, validators, canDelete, destroy
- Make internal functions private in thinking-utils.js
- Remove commented-out code from constants.js
- Remove deprecated .glass-panel CSS class
- Add silent handler for Claude Code event logging (/api/event_logging/batch)
- Suppress logging for /v1/messages/count_tokens (501 responses)
- Fix catch-all to use originalUrl (wildcard strips req.path)

Co-Authored-By: Claude <noreply@anthropic.com>

src/constants.js

@@ -67,17 +67,7 @@ export const LOAD_CODE_ASSIST_ENDPOINTS = [
 // Endpoint order for onboardUser (same as generateContent fallbacks)
 export const ONBOARD_USER_ENDPOINTS = ANTIGRAVITY_ENDPOINT_FALLBACKS;
 
-// Hybrid headers specifically for loadCodeAssist
-// Uses google-api-nodejs-client User-Agent (required for project discovery on some accounts)
-// export const LOAD_CODE_ASSIST_HEADERS = {
-//     'User-Agent': 'google-api-nodejs-client/9.15.1',
-//     'X-Goog-Api-Client': 'google-cloud-sdk vscode_cloudshelleditor/0.1',
-//     'Client-Metadata': JSON.stringify({
-//         ideType: 'IDE_UNSPECIFIED',
-//         platform: 'PLATFORM_UNSPECIFIED',
-//         pluginType: 'GEMINI'
-//     })
-// };
+// Headers for loadCodeAssist API
 export const LOAD_CODE_ASSIST_HEADERS = ANTIGRAVITY_HEADERS;
 
 // Default project ID if none can be discovered