Add ability to add/remove custom tickers (e.g. EURCHF=X) on the Markets & Prices page without editing config. Tickers are validated via yfinance, persisted in market_watchlist SQLite table, merged into the quotes feed as a 'custom' group, and shown in a dedicated tab with per-card remove buttons. Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>