feat(ui): auto-refresh chat on event gap with visual marker

[brianleach]

Summary

• When the web UI detects an event sequence gap (common after returning from a backgrounded tab), automatically reload chat history instead of showing a "refresh recommended" error
• A visual gap divider marks where new messages begin, showing the count of messages that arrived while the user was away
• The gap marker can be dismissed via its X button and is automatically cleared on manual refresh, session switch, or reconnect

Details

Before: gap detection set lastError with "event gap detected (expected seq X, got Y); refresh recommended" — user had to manually click refresh.

After: gap triggers loadChatHistory() automatically. If new messages appeared, an accent-colored divider is inserted at the boundary: "N new messages since you were away".

Files changed

File	Change
ui/src/ui/app-gateway.ts	Auto-refresh on gap instead of setting error
ui/src/ui/views/chat.ts	Render gap divider + dismiss button
ui/src/ui/types/chat-types.ts	New "gap" ChatItem kind
ui/src/styles/chat/grouped.css	Gap divider styling (accent color)
ui/src/ui/app.ts	chatGapIndex state
ui/src/ui/app-view-state.ts	Type for chatGapIndex
ui/src/ui/app-render.ts	Pass gap props to chat view
ui/src/ui/app-render.helpers.ts	Clear gap on manual refresh
ui/src/ui/app-gateway.node.test.ts	Updated + new tests, fixed Node 22 localStorage

Test plan

• Open web UI, send a few messages, then background the tab
• Trigger activity from another channel (Telegram, cron, etc.) so events are missed
• Return to the tab — verify chat auto-refreshes and gap divider appears
• Verify gap divider shows correct new message count
• Click X on gap divider — verify it dismisses
• Click manual refresh button — verify gap clears
• Switch sessions — verify gap clears
• Disconnect/reconnect — verify gap clears
• Run ui/src/ui/app-gateway.node.test.ts — all 10 tests pass