FEAT: Email search functionality

[andrinoff]

Is your feature request related to a problem?

There is no way to search emails in matcha. Users with large mailboxes have no option other than scrolling to find specific emails. Search is a fundamental email client feature.

Describe the solution you'd like

1. Add a Search(ctx context.Context, query string, folder string) ([]Email, error) method to the backend.Provider interface
2. Implement using IMAP SEARCH command for the IMAP backend
3. Implement using JMAP Email/query filters for the JMAP backend
4. Add a search TUI overlay (e.g. triggered by / key) with a text input and results list
5. Support searching by sender, subject, body text, and date range

Describe alternatives you've considered

Client-side filtering of already-fetched emails. This would be simpler but only works for cached emails and doesn't scale to large mailboxes.

Additional context

Both IMAP and JMAP protocols have native server-side search capabilities that are already available in the protocol libraries used by matcha (go-imap/v2 and go-jmap) but are not currently exposed.

[andrinoff]

CONTEXT:

There is fuzzy search, but it only works for the emails loaded, it doesnt search further, than what there already is

[mvanhorn]

Implemented in #1186. Added EmailSearcher interface, IMAP SEARCH/ESEARCH (RFC 4731), JMAP Email/query, POP3 ErrNotSupported, and a /-triggered TUI overlay. Touches the same DSL keys called out in #1131; partial overlap there.