feat: add chat read/unread indicator to sidebar (#23129)

## Summary

Adds read/unread tracking for chats so users can see which agent
conversations have new assistant messages they haven't viewed.

## Backend Changes

- Adds `last_read_message_id` column to the `chats` table (migration
000439).
- Computes `has_unread` as a virtual column in `GetChatsByOwnerID` using
an `EXISTS` subquery checking for assistant messages beyond the read
cursor.
- Exposes `has_unread` on the `codersdk.Chat` struct and auto-generated
TypeScript types.
- Updates `last_read_message_id` on stream connect/disconnect in
`streamChat`, avoiding per-message API calls during active streaming.
- Uses `context.WithoutCancel` for the deferred disconnect write so the
DB update succeeds even after the client disconnects.

## Frontend Changes

- Bold title (`font-semibold`) for unread chats in the sidebar.
- Small blue dot indicator next to the relative timestamp.
- Suppresses unread indicator for the currently active chat via
`isActive` from NavLink.

## Design Decisions

- Only `assistant` messages count as unread — the user's own messages
don't trigger the indicator.
- No foreign key on `last_read_message_id` since messages can be deleted
(via rollback/truncation) and the column is just a high-water mark.
- Zero API calls during streaming: exactly 2 DB writes per stream
session (connect + disconnect).
- Unread state refreshes on chat list load and window focus. The
`watchChats` WebSocket optimistically marks non-active chats as unread
on `status_change` events, but does not carry a server-computed
`has_unread` field. Navigating to a chat optimistically clears its
unread indicator in the cache.

codersdk/chats.go

@@ -64,6 +64,10 @@ type Chat struct {
 	PinOrder          int32             `json:"pin_order"`
 	MCPServerIDs      []uuid.UUID       `json:"mcp_server_ids" format:"uuid"`
 	Labels            map[string]string `json:"labels"`
+	// HasUnread is true when assistant messages exist beyond
+	// the owner's read cursor, which updates on stream
+	// connect and disconnect.
+	HasUnread bool `json:"has_unread"`
 }
 
 // ChatMessage represents a single message in a chat.