feat: folders (#218)

[andrinoff]

Replaces the hardcoded mailbox system (Inbox/Sent/Trash/Archive) with a dynamic folder sidebar that displays all IMAP folders across all accounts.

Closes #218

Changes

Folder sidebar

• New FolderInbox component with a folder list on the left and email list on the right
• Tab/Shift+Tab to switch between folders
• Folders fetched from all accounts and merged (deduplicated)
• INBOX always sorted to the top, rest alphabetical
• Active folder highlighted with background color
• [Gmail]/ and [Google Mail]/ prefixes stripped from display names

Move to folder (m)

• Press m on a selected email to open a folder picker overlay
• Navigate with j/k, confirm with Enter, cancel with Esc
• Moves the email via IMAP and removes it from the current view

Caching

• Folder lists cached per account to ~/.config/matcha/folder_cache.json
• Folder emails cached per folder to ~/.config/matcha/folder_emails/<folder>.json
• On startup: cached folders and INBOX emails display instantly, background refresh fetches latest
• On folder switch: in-memory cache used first, then disk cache, then IMAP fetch
• Pagination results persisted to disk cache so switching back to a folder preserves loaded emails

Pagination

• Per-account exhaustion tracking prevents infinite "loading more..." on folders with few emails
• Increased initial fetch and pagination limits from 20 to 50 per account
• Stale fetch responses ignored when user has already switched to a different folder

Cleanup

• Removed "Sent" and "Trash & Archive" from the Choice menu (all folders now accessible via sidebar)
• Simplified email helpers in main model to work with folder-based system