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.

coderd/database/modelqueries.go

@@ -741,10 +741,10 @@ func (q *sqlQuerier) CountAuthorizedConnectionLogs(ctx context.Context, arg Coun
 }
 
 type chatQuerier interface {
-	GetAuthorizedChats(ctx context.Context, arg GetChatsParams, prepared rbac.PreparedAuthorized) ([]Chat, error)
+	GetAuthorizedChats(ctx context.Context, arg GetChatsParams, prepared rbac.PreparedAuthorized) ([]GetChatsRow, error)
 }
 
-func (q *sqlQuerier) GetAuthorizedChats(ctx context.Context, arg GetChatsParams, prepared rbac.PreparedAuthorized) ([]Chat, error) {
+func (q *sqlQuerier) GetAuthorizedChats(ctx context.Context, arg GetChatsParams, prepared rbac.PreparedAuthorized) ([]GetChatsRow, error) {
 	authorizedFilter, err := prepared.CompileToSQL(ctx, rbac.ConfigChats())
 	if err != nil {
 		return nil, xerrors.Errorf("compile authorized filter: %w", err)
@@ -769,32 +769,33 @@ func (q *sqlQuerier) GetAuthorizedChats(ctx context.Context, arg GetChatsParams,
 		return nil, err
 	}
 	defer rows.Close()
-	var items []Chat
+	var items []GetChatsRow
 	for rows.Next() {
-		var i Chat
+		var i GetChatsRow
 		if err := rows.Scan(
-			&i.ID,
-			&i.OwnerID,
-			&i.WorkspaceID,
-			&i.Title,
-			&i.Status,
-			&i.WorkerID,
-			&i.StartedAt,
-			&i.HeartbeatAt,
-			&i.CreatedAt,
-			&i.UpdatedAt,
-			&i.ParentChatID,
-			&i.RootChatID,
-			&i.LastModelConfigID,
-			&i.Archived,
-			&i.LastError,
-			&i.Mode,
-			pq.Array(&i.MCPServerIDs),
-			&i.Labels,
-			&i.BuildID,
-			&i.AgentID,
-			&i.PinOrder,
-		); err != nil {
+			&i.Chat.ID,
+			&i.Chat.OwnerID,
+			&i.Chat.WorkspaceID,
+			&i.Chat.Title,
+			&i.Chat.Status,
+			&i.Chat.WorkerID,
+			&i.Chat.StartedAt,
+			&i.Chat.HeartbeatAt,
+			&i.Chat.CreatedAt,
+			&i.Chat.UpdatedAt,
+			&i.Chat.ParentChatID,
+			&i.Chat.RootChatID,
+			&i.Chat.LastModelConfigID,
+			&i.Chat.Archived,
+			&i.Chat.LastError,
+			&i.Chat.Mode,
+			pq.Array(&i.Chat.MCPServerIDs),
+			&i.Chat.Labels,
+			&i.Chat.BuildID,
+			&i.Chat.AgentID,
+			&i.Chat.PinOrder,
+			&i.Chat.LastReadMessageID,
+			&i.HasUnread); err != nil {
 			return nil, err
 		}
 		items = append(items, i)