feat(auth): add /api/me endpoint, avatar upload, and profile picture support

- Add migration 005: nullable avatar_url column on users table
- Add GET /api/me: returns id, username, avatar_url from DB (fixes UUID-on-profile bug)
- Add PUT /api/me/avatar: accepts raw image bytes (≤1 MB, jpeg/png/webp/gif),
  writes to avatars/ dir, updates avatar_url in DB
- Serve /avatars via ServeDir so uploaded images are publicly accessible
- Update account.html: fetch username from /api/me instead of parsing JWT;
  add circular avatar display with initials fallback and click-to-upload
- Add SolitaireServerClient::fetch_me() for desktop/Android profile display
- Add avatar_url field to SyncBackend::SolitaireServer settings (serde default None)
- Update sqlx offline query cache for new avatar_url queries

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

solitaire_server/migrations/005_avatar.sql

@@ -0,0 +1,4 @@
+-- Migration 005: user avatar
+-- Adds a nullable avatar_url column to users.
+-- Existing rows receive NULL (no avatar set).
+ALTER TABLE users ADD COLUMN avatar_url TEXT;