[FEATURE] feat: Auto-save message drafts per room

[deepak0x]

As an EmbeddedChat developer

I need to:
Have my in-progress message automatically saved as a draft when I navigate away from a room or close the chat, and have it restored when I return to that room.

So That:
I never lose text I was composing mid-thought, and I can confidently switch between rooms without finishing every message before moving on.

Acceptance Criteria

• Unsent text in the message composer is automatically persisted as a draft for the active room whenever the user navigates away, switches rooms, or closes the widget.
• When the user returns to a room that has a saved draft, the composer is pre-populated with the draft text.
• Drafts survive a full page reload.
• A visible 'Draft' label appears in the composer (or near the room entry in any room list) when a draft exists for that room, so the user knows at a glance.
• Sending a message clears the draft for that room.
• The user can manually discard a draft (e.g. clearing the composer dismisses it).
• Draft saving respects the currently configured room/channel so drafts are never mixed across rooms.
• The feature works correctly in both single-room and multi-room (sidebar) configurations.

[deepak0x]

cc @Spiral-Memory — happy to implement this if it fits the roadmap.