feat: add chat summary tab in the right sidebar and per-chat cost endpoint (#26649)

Stacked on #26657 (the persisted whole-chat summary backend). Base
branch is `chat-summary-62j9`; review/merge that first.

Adds a reusable `ChatSummary` component.

The summary text is the persisted whole-chat summary (`chat.summary`)
introduced by #26657. It is generated asynchronously and may be `null`
until the first summary is produced, in which case the popover renders a
muted empty state. Live updates arrive via that PR's
`chat_summary_change` watch event, which is already merged into the chat
caches.

Cost is served by a new per-chat endpoint, `GET
/api/experimental/chats/{chat}/cost`, which rolls up assistant-message
cost across a chat's root and child (subagent) chats and is authorized
like the other `{chat}` routes (read on the chat, 404 otherwise).

Visual and interaction coverage lives in `ChatSummary.stories.tsx` and
`ChatSummaryPopover.stories.tsx` (including populated-summary,
empty-state, and cost-loading cases).

---------

Co-authored-by: Cursor <cursoragent@cursor.com>

coderd/database/dbmetrics/querymetrics.go

@@ -1713,6 +1713,14 @@ func (m queryMetricsStore) GetChatModelConfigsForTelemetry(ctx context.Context)
 	return r0, r1
 }
 
+func (m queryMetricsStore) GetChatModelUsageCostByChatID(ctx context.Context, chatID uuid.UUID) (database.GetChatModelUsageCostByChatIDRow, error) {
+	start := time.Now()
+	r0, r1 := m.s.GetChatModelUsageCostByChatID(ctx, chatID)
+	m.queryLatencies.WithLabelValues("GetChatModelUsageCostByChatID").Observe(time.Since(start).Seconds())
+	m.queryCounts.WithLabelValues(httpmw.ExtractHTTPRoute(ctx), httpmw.ExtractHTTPMethod(ctx), "GetChatModelUsageCostByChatID").Inc()
+	return r0, r1
+}
+
 func (m queryMetricsStore) GetChatPersonalModelOverridesEnabled(ctx context.Context) (bool, error) {
 	start := time.Now()
 	r0, r1 := m.s.GetChatPersonalModelOverridesEnabled(ctx)