RW mode: full event editing

[Phoenix-]

Turn Meridian from a read-only viewer into a real calendar editor.

Tracked as an epic — will spawn sub-issues once we get here. Listed without milestone because this is post-v1.0.

Surface area (rough)

• Create / edit / delete events (single + recurring).
• Recurrence editing semantics (this-only / this-and-future / all).
• Drag to move/resize in month and agenda views.
• Conflict resolution when the server changes the event under us (ETag mismatch).
• Optimistic UI vs sync-on-save — pick one and stick with it.
• Undo for destructive actions.
• Attendee invite/RSVP (depends on Event Details: Guests Event details: GMeet link + Guests #6 landing first).

Status

Epic / not started. Open sub-issues when work begins.