Title: Add "Mark All as Read" for Entire Mailbox (including imported emails)

Description:
After importing from Gmail, I have 3000+ unread emails. The only way to mark
them as read is to go through 60+ pages of pagination (50 emails/page), which
is impractical.

Feature Request:
- Add a "Mark all emails in folder/mailbox as read" button in the web UI
- Allow bulk operations via IMAP/API for programmatic access
- Consider applying this to all categories, not just folders -> This is very important, as my inbox is now a jumble of everything I had in the right gmail categories, quite annoying!

Impact:
- Critical for users migrating from other email providers
- Existing feature request: https://protonmail.uservoice.com/... (reference if exists)

Current Workaround:
Requires third-party Python script via Proton Bridge. Should be a native
feature in the web interface.