Implement the following plan:

# Plan: Transform Anky into a Farcaster MiniApp

## Context

Anky currently authenticates users via Privy (wallet) and X OAuth. The goal is to make Anky work as a **Farcaster MiniApp** — when opened inside Warpcast/Farcaster clients, it authenticates via the Farcaster SDK, gets the user's FID, profile, and embedded wallet automatically. No login page needed. The FID becomes the atomic unit of identity.

The app already stores `farcaster_fid`, `farcaster_username`, and `farcaster_pfp_url` on users (via Neynar lookup during Privy login). This integration deepens that — Farcaster becomes a first-class auth path.

## What Changes

### 1. Serve the Farcaster manifest at `/.well-known/farcaster.json`
### 2. Add `fc:miniapp` meta tag to `templates/base.html`
### 3. Add Farcaster SDK initialization to `templates/base.html`
### 4. Add Farcaster auth endpoint: `POST /auth/farcaster/verify`
### 5. Get wallet address from Farcaster embedded wallet
### 6. Conditional UI: hide login navbar items when in Farcaster