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*

coderd/database/dbauthz/dbauthz.go

@@ -5523,6 +5523,17 @@ func (q *querier) PaginatedOrganizationMembers(ctx context.Context, arg database
 	return q.db.PaginatedOrganizationMembers(ctx, arg)
 }
 
+func (q *querier) PinChatByID(ctx context.Context, id uuid.UUID) error {
+	chat, err := q.db.GetChatByID(ctx, id)
+	if err != nil {
+		return err
+	}
+	if err := q.authorizeContext(ctx, policy.ActionUpdate, chat); err != nil {
+		return err
+	}
+	return q.db.PinChatByID(ctx, id)
+}
+
 func (q *querier) PopNextQueuedMessage(ctx context.Context, chatID uuid.UUID) (database.ChatQueuedMessage, error) {
 	chat, err := q.db.GetChatByID(ctx, chatID)
 	if err != nil {
@@ -5648,6 +5659,17 @@ func (q *querier) UnfavoriteWorkspace(ctx context.Context, id uuid.UUID) error {
 	return update(q.log, q.auth, fetch, q.db.UnfavoriteWorkspace)(ctx, id)
 }
 
+func (q *querier) UnpinChatByID(ctx context.Context, id uuid.UUID) error {
+	chat, err := q.db.GetChatByID(ctx, id)
+	if err != nil {
+		return err
+	}
+	if err := q.authorizeContext(ctx, policy.ActionUpdate, chat); err != nil {
+		return err
+	}
+	return q.db.UnpinChatByID(ctx, id)
+}
+
 func (q *querier) UnsetDefaultChatModelConfigs(ctx context.Context) error {
 	if err := q.authorizeContext(ctx, policy.ActionUpdate, rbac.ResourceSystem); err != nil {
 		return err
@@ -5748,6 +5770,17 @@ func (q *querier) UpdateChatModelConfig(ctx context.Context, arg database.Update
 	return q.db.UpdateChatModelConfig(ctx, arg)
 }
 
+func (q *querier) UpdateChatPinOrder(ctx context.Context, arg database.UpdateChatPinOrderParams) error {
+	chat, err := q.db.GetChatByID(ctx, arg.ID)
+	if err != nil {
+		return err
+	}
+	if err := q.authorizeContext(ctx, policy.ActionUpdate, chat); err != nil {
+		return err
+	}
+	return q.db.UpdateChatPinOrder(ctx, arg)
+}
+
 func (q *querier) UpdateChatProvider(ctx context.Context, arg database.UpdateChatProviderParams) (database.ChatProvider, error) {
 	if err := q.authorizeContext(ctx, policy.ActionUpdate, rbac.ResourceDeploymentConfig); err != nil {
 		return database.ChatProvider{}, err

coderd/database/dbauthz/dbauthz_test.go

@@ -401,6 +401,18 @@ func (s *MethodTestSuite) TestChats() {
 		dbm.EXPECT().UnarchiveChatByID(gomock.Any(), chat.ID).Return(nil).AnyTimes()
 		check.Args(chat.ID).Asserts(chat, policy.ActionUpdate).Returns()
 	}))
+	s.Run("PinChatByID", s.Mocked(func(dbm *dbmock.MockStore, faker *gofakeit.Faker, check *expects) {
+		chat := testutil.Fake(s.T(), faker, database.Chat{})
+		dbm.EXPECT().GetChatByID(gomock.Any(), chat.ID).Return(chat, nil).AnyTimes()
+		dbm.EXPECT().PinChatByID(gomock.Any(), chat.ID).Return(nil).AnyTimes()
+		check.Args(chat.ID).Asserts(chat, policy.ActionUpdate).Returns()
+	}))
+	s.Run("UnpinChatByID", s.Mocked(func(dbm *dbmock.MockStore, faker *gofakeit.Faker, check *expects) {
+		chat := testutil.Fake(s.T(), faker, database.Chat{})
+		dbm.EXPECT().GetChatByID(gomock.Any(), chat.ID).Return(chat, nil).AnyTimes()
+		dbm.EXPECT().UnpinChatByID(gomock.Any(), chat.ID).Return(nil).AnyTimes()
+		check.Args(chat.ID).Asserts(chat, policy.ActionUpdate).Returns()
+	}))
 	s.Run("SoftDeleteChatMessagesAfterID", s.Mocked(func(dbm *dbmock.MockStore, faker *gofakeit.Faker, check *expects) {
 		chat := testutil.Fake(s.T(), faker, database.Chat{})
 		arg := database.SoftDeleteChatMessagesAfterIDParams{
@@ -827,6 +839,16 @@ func (s *MethodTestSuite) TestChats() {
 		dbm.EXPECT().UpdateChatProvider(gomock.Any(), arg).Return(provider, nil).AnyTimes()
 		check.Args(arg).Asserts(rbac.ResourceDeploymentConfig, policy.ActionUpdate).Returns(provider)
 	}))
+	s.Run("UpdateChatPinOrder", s.Mocked(func(dbm *dbmock.MockStore, faker *gofakeit.Faker, check *expects) {
+		chat := testutil.Fake(s.T(), faker, database.Chat{})
+		arg := database.UpdateChatPinOrderParams{
+			ID:       chat.ID,
+			PinOrder: 2,
+		}
+		dbm.EXPECT().GetChatByID(gomock.Any(), chat.ID).Return(chat, nil).AnyTimes()
+		dbm.EXPECT().UpdateChatPinOrder(gomock.Any(), arg).Return(nil).AnyTimes()
+		check.Args(arg).Asserts(chat, policy.ActionUpdate).Returns()
+	}))
 	s.Run("UpdateChatStatus", s.Mocked(func(dbm *dbmock.MockStore, faker *gofakeit.Faker, check *expects) {
 		chat := testutil.Fake(s.T(), faker, database.Chat{})
 		arg := database.UpdateChatStatusParams{