refactor: Reorganize src/ into modular folder structure

Split large monolithic files into focused modules:
- cloudcode-client.js (1,107 lines) → src/cloudcode/ (9 files)
- account-manager.js (639 lines) → src/account-manager/ (5 files)
- Move auth files to src/auth/ (oauth, token-extractor, database)
- Move CLI to src/cli/accounts.js

Update all import paths and documentation.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

src/server.js

@@ -6,10 +6,10 @@
 
 import express from 'express';
 import cors from 'cors';
-import { sendMessage, sendMessageStream, listModels, getModelQuotas } from './cloudcode-client.js';
-import { forceRefresh } from './token-extractor.js';
+import { sendMessage, sendMessageStream, listModels, getModelQuotas } from './cloudcode/index.js';
+import { forceRefresh } from './auth/token-extractor.js';
 import { REQUEST_BODY_LIMIT } from './constants.js';
-import { AccountManager } from './account-manager.js';
+import { AccountManager } from './account-manager/index.js';
 import { formatDuration } from './utils/helpers.js';
 import { logger } from './utils/logger.js';