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/coderd.go

@@ -1420,6 +1420,7 @@ func New(options *Options) *API {
 				})
 				r.Get("/", api.getChat)
 				r.Patch("/", api.patchChat)
+				r.Get("/cost", api.getChatCost)
 				r.Get("/messages", api.getChatMessages)
 				r.Post("/messages", api.postChatMessages)
 				r.Patch("/messages/{message}", api.patchChatMessage)