fix: auto-scroll to bottom when sending messages

[TigerInYourDream]

Summary

When a user sends a message (text or location), the timeline should automatically scroll to the bottom so the sent message is visible — matching the behavior of WeChat, Telegram, and other chat apps.

Approach

• Introduces a MessageAction::MessageSubmittedLocally action, emitted directly from room_input_bar.rs at send time
• room_screen.rs handles this action by scrolling to the last timeline item and enabling tail-range mode
• Replaces the previous indirect approach of checking if the last appended timeline item's sender matches the current user

Why this approach is better

Aspect	Old (sender matching in NewItems)	New (MessageSubmittedLocally action)
Timing	Waits for async timeline echo-back	Fires immediately on send
Reliability	Depends on timeline update arrival	Always fires, no race conditions
Signal source	Indirect inference (match sender)	Direct intent ("I just sent a message")
Code separation	Mixed into process_timeline_updates()	Emit in input bar, handle in room screen
Makepad pattern	Ad-hoc	Follows cx.widget_action() communication pattern

Changes

• src/room/room_input_bar.rs: Emit MessageSubmittedLocally after both location send and text send
• src/home/room_screen.rs: Handle the action with set_first_id_and_scroll() + set_tail_range(true), remove old sender-matching logic

Test plan

• Send a text message while scrolled to bottom → timeline stays at bottom, new message visible
• Scroll up, then send a text message → timeline auto-scrolls to bottom, new message visible
• Send a location message → same auto-scroll behavior
• Receive a message from another user while scrolled up → should NOT auto-scroll (unchanged behavior)