Add a Drafts email support to message center #318

Problem
When composing a bulk email, there's no way to save it for later. You either send it (or schedule it) right away, or you lose your progress if you step away.

Description
Other message centers in eventyay already let organizers save an in-progress email as a draft and pick it back up whenever they're ready, with everything — recipients, subject, body, filters — exactly as they left it. Exhibition's own message center should offer the same convenience.

Current Behavior

• The compose screen only supports "send now" or "schedule for later."
• A saved-but-unsent email looks identical to a scheduled email with no time set — there's no real draft state.
• There's no dedicated Drafts tab, so a half-written email isn't separated from emails about to go out.

Expected Behavior

• Organizers can explicitly save a composed email as a draft at any point.
• A dedicated Drafts tab shows these, separate from Outbox and Sent.
• Reopening a draft restores it fully for continued editing, until it's sent or moved to the outbox.

Acceptance Criteria

• A composed email can be saved as a draft (distinct from "scheduled, no date")
• Drafts appear in their own list, separate from Outbox and Sent
• Reopening a draft restores full state for editing
• A draft is never sent by accident
• Existing scheduled/outbox behavior keeps working as-is