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/dbmock/dbmock.go

@@ -1804,10 +1804,10 @@ func (mr *MockStoreMockRecorder) GetAuthorizedAuditLogsOffset(ctx, arg, prepared
 }
 
 // GetAuthorizedChats mocks base method.
-func (m *MockStore) GetAuthorizedChats(ctx context.Context, arg database.GetChatsParams, prepared rbac.PreparedAuthorized) ([]database.Chat, error) {
+func (m *MockStore) GetAuthorizedChats(ctx context.Context, arg database.GetChatsParams, prepared rbac.PreparedAuthorized) ([]database.GetChatsRow, error) {
 	m.ctrl.T.Helper()
 	ret := m.ctrl.Call(m, "GetAuthorizedChats", ctx, arg, prepared)
-	ret0, _ := ret[0].([]database.Chat)
+	ret0, _ := ret[0].([]database.GetChatsRow)
 	ret1, _ := ret[1].(error)
 	return ret0, ret1
 }
@@ -2344,10 +2344,10 @@ func (mr *MockStoreMockRecorder) GetChatWorkspaceTTL(ctx any) *gomock.Call {
 }
 
 // GetChats mocks base method.
-func (m *MockStore) GetChats(ctx context.Context, arg database.GetChatsParams) ([]database.Chat, error) {
+func (m *MockStore) GetChats(ctx context.Context, arg database.GetChatsParams) ([]database.GetChatsRow, error) {
 	m.ctrl.T.Helper()
 	ret := m.ctrl.Call(m, "GetChats", ctx, arg)
-	ret0, _ := ret[0].([]database.Chat)
+	ret0, _ := ret[0].([]database.GetChatsRow)
 	ret1, _ := ret[1].(error)
 	return ret0, ret1
 }
@@ -7775,6 +7775,20 @@ func (mr *MockStoreMockRecorder) UpdateChatLastModelConfigByID(ctx, arg any) *go
 	return mr.mock.ctrl.RecordCallWithMethodType(mr.mock, "UpdateChatLastModelConfigByID", reflect.TypeOf((*MockStore)(nil).UpdateChatLastModelConfigByID), ctx, arg)
 }
 
+// UpdateChatLastReadMessageID mocks base method.
+func (m *MockStore) UpdateChatLastReadMessageID(ctx context.Context, arg database.UpdateChatLastReadMessageIDParams) error {
+	m.ctrl.T.Helper()
+	ret := m.ctrl.Call(m, "UpdateChatLastReadMessageID", ctx, arg)
+	ret0, _ := ret[0].(error)
+	return ret0
+}
+
+// UpdateChatLastReadMessageID indicates an expected call of UpdateChatLastReadMessageID.
+func (mr *MockStoreMockRecorder) UpdateChatLastReadMessageID(ctx, arg any) *gomock.Call {
+	mr.mock.ctrl.T.Helper()
+	return mr.mock.ctrl.RecordCallWithMethodType(mr.mock, "UpdateChatLastReadMessageID", reflect.TypeOf((*MockStore)(nil).UpdateChatLastReadMessageID), ctx, arg)
+}
+
 // UpdateChatMCPServerIDs mocks base method.
 func (m *MockStore) UpdateChatMCPServerIDs(ctx context.Context, arg database.UpdateChatMCPServerIDsParams) (database.Chat, error) {
 	m.ctrl.T.Helper()