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/apidoc/docs.go

@@ -580,6 +580,42 @@ const docTemplate = `{
                 ]
             }
         },
+        "/api/experimental/chats/{chat}/cost": {
+            "get": {
+                "description": "Experimental: this endpoint is subject to change.",
+                "produces": [
+                    "application/json"
+                ],
+                "tags": [
+                    "Chats"
+                ],
+                "summary": "Get chat cost",
+                "operationId": "get-chat-cost",
+                "parameters": [
+                    {
+                        "type": "string",
+                        "format": "uuid",
+                        "description": "Chat ID",
+                        "name": "chat",
+                        "in": "path",
+                        "required": true
+                    }
+                ],
+                "responses": {
+                    "200": {
+                        "description": "OK",
+                        "schema": {
+                            "$ref": "#/definitions/codersdk.ChatCost"
+                        }
+                    }
+                },
+                "security": [
+                    {
+                        "CoderSessionToken": []
+                    }
+                ]
+            }
+        },
         "/api/experimental/chats/{chat}/diff": {
             "get": {
                 "description": "Experimental: this endpoint is subject to change.",
@@ -17271,6 +17307,24 @@ const docTemplate = `{
                 }
             }
         },
+        "codersdk.ChatCost": {
+            "type": "object",
+            "properties": {
+                "chat_id": {
+                    "type": "string",
+                    "format": "uuid"
+                },
+                "priced_message_count": {
+                    "type": "integer"
+                },
+                "total_cost_micros": {
+                    "type": "integer"
+                },
+                "unpriced_messages_having_usage_count": {
+                    "type": "integer"
+                }
+            }
+        },
         "codersdk.ChatDiffContents": {
             "type": "object",
             "properties": {