feat: add pinned chats with drag-to-reorder (#23615)

https://github.com/user-attachments/assets/bd5d12a1-61b3-4b7d-83b6-317bdfb60b3c

## Summary

Adds pinned chats to the agents page sidebar with server-side
persistence and drag-to-reorder. Users can pin/unpin chats via the
context menu, and pinned chats appear in a dedicated "Pinned" section
above the time-grouped list.

## Database

Migration `000453_chat_pin_order`: adds `pin_order integer DEFAULT 0 NOT
NULL` column on `chats` (0 = unpinned, 1+ = pinned in display order).
Three SQL queries handle pin operations server-side using CTEs with
`ROW_NUMBER()`:

- `PinChatByID`: normalizes existing orders and appends to end
- `UnpinChatByID`: sets target to 0 and compacts remaining pins
- `UpdateChatPinOrder`: shifts neighbors, clamps to `[1, pinned_count]`

All queries exclude archived chats. `ArchiveChatByID` clears `pin_order`
on archive. The handler rejects pinning archived chats with 400.

## Backend

Pin/unpin/reorder go through the existing `PATCH
/api/experimental/chats/{chat}` via the `pin_order` field on
`UpdateChatRequest`. The handler routes based on current pin state:
`pin_order == 0` unpins, `> 0` on an already-pinned chat reorders, `> 0`
on an unpinned chat appends to end.

## Frontend

- `pinChat` / `unpinChat` / `reorderPinnedChat` optimistic mutations
using shared `isChatListQuery` predicate
- Sidebar renders Pinned section above time groups, excludes pinned
chats from time groups
- Pin/Unpin context menu items (hidden for child/delegated chats)
- `@dnd-kit/core` + `@dnd-kit/sortable` for drag-to-reorder with
`MouseSensor`, `TouchSensor`, and `KeyboardSensor`
- Local pin-order override prevents flash on drop; click blocker
prevents NavLink navigation after drag

---
*PR generated with Coder Agents*

codersdk/chats.go

@@ -61,6 +61,7 @@ type Chat struct {
 	CreatedAt         time.Time         `json:"created_at" format:"date-time"`
 	UpdatedAt         time.Time         `json:"updated_at" format:"date-time"`
 	Archived          bool              `json:"archived"`
+	PinOrder          int32             `json:"pin_order"`
 	MCPServerIDs      []uuid.UUID       `json:"mcp_server_ids" format:"uuid"`
 	Labels            map[string]string `json:"labels"`
 }
@@ -323,8 +324,18 @@ type CreateChatRequest struct {
 
 // UpdateChatRequest is the request to update a chat.
 type UpdateChatRequest struct {
-	Title    *string            `json:"title,omitempty"`
-	Archived *bool              `json:"archived,omitempty"`
+	Title    *string `json:"title,omitempty"`
+	Archived *bool   `json:"archived,omitempty"`
+	// PinOrder controls the chat's pinned state and position.
+	// - nil: no change to pin state.
+	// - 0: unpin the chat.
+	// - >0 (chat is unpinned): pin the chat, appending it to
+	//   the end of the pinned list. The specific value is
+	//   ignored; the server assigns the next available position.
+	// - >0 (chat is already pinned): move the chat to the
+	//   requested position, shifting neighbors as needed. The
+	//   value is clamped to [1, pinned_count].
+	PinOrder *int32             `json:"pin_order,omitempty"`
 	Labels   *map[string]string `json:"labels,omitempty"`
 }