Describe the feature

A first-class email client lets you find any message in seconds. Right now matcha can only filter what's loaded into the current folder view.

Ship a local full-text search index (SQLite FTS5 or Bleve) that:

• ingests every fetched message body + headers
• indexes per-account, per-folder
• supports prefix, phrase, and field-qualified queries (from:alice subject:invoice)
• updates incrementally on IDLE notifications
• exposes a TUI search modal (/) and a matcha search CLI command

Why this matters

Search is table stakes for any mail client; Mutt, aerc, neomutt, Thunderbird all have it. Without it the user has to fall back to grepping raw IMAP downloads.

Notes

• SQLite FTS5 is probably the cheapest path: zero deps, atomic writes, ~tens of MB for a heavy inbox.
• Index should live in ~/.cache/matcha/search.db so it can be rebuilt without losing config.
• Tokenizer needs to handle CJK / Cyrillic / accented Latin reasonably (FTS5 unicode61 tokenizer).