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/querier.go

@@ -456,6 +456,11 @@ type sqlcQuerier interface {
 	// Returns all model configurations for telemetry snapshot collection.
 	// deleted = false guarantees ai_provider_id is non-null, so INNER JOIN is safe.
 	GetChatModelConfigsForTelemetry(ctx context.Context) ([]GetChatModelConfigsForTelemetryRow, error)
+	// Assistant-message cost rolled up over the requested chat's subtree: the
+	// chat itself plus every descendant reachable through parent_chat_id. A
+	// root chat therefore reports its whole tree, while a subagent chat
+	// reports only its own spend plus any nested subagents it spawned.
+	GetChatModelUsageCostByChatID(ctx context.Context, chatID uuid.UUID) (GetChatModelUsageCostByChatIDRow, error)
 	// GetChatPersonalModelOverridesEnabled returns whether users may configure
 	// personal chat model overrides. It defaults to false when unset.
 	GetChatPersonalModelOverridesEnabled(ctx context.Context) (bool, error)