fix: move API keys to .env.local, remove localStorage dual system

Single source of truth: OPENAI_API_KEY in .env.local. Deleted the
apiKeyService class/singleton that stored keys in localStorage.
All server code reads process.env directly. FirstRunModal and
Settings now show .env.local instructions instead of key input.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/services/database/dimensionService.ts

@@ -1,7 +1,7 @@
 import { getSQLiteClient } from './sqlite-client';
 import { openai as openaiProvider } from '@ai-sdk/openai';
 import { generateText } from 'ai';
-import { hasValidOpenAiKey } from './descriptionService';
+import { hasValidOpenAiKey } from '../storage/apiKeys';
 
 export interface Dimension {
   name: string;