Frontend claims standalone functionality but depends on backend API (breaking core features) #535

### Description:
The README states that the frontend can run independently by opening index.html. However, core features like AI-powered recommendations depend on backend endpoints (/api/v1/generate-note).

This leads to broken functionality when running the frontend alone:

API calls fail
No fallback or error messaging
Users receive no response

Expected Behavior:

Either frontend works independently (with mock/fallback data)
OR clear error message + setup instructions

### Proposed Solution:

Add API availability check
Provide fallback/mock responses
Add UI error messaging when backend is unavailable

Tech Areas:

frontend/js/app.js
config.js

Impact:

Fixes broken user experience
Improves onboarding for contributors
Aligns README with actual behavior