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/dbmock/dbmock.go

@@ -7411,6 +7411,20 @@ func (mr *MockStoreMockRecorder) PaginatedOrganizationMembers(ctx, arg any) *gom
 	return mr.mock.ctrl.RecordCallWithMethodType(mr.mock, "PaginatedOrganizationMembers", reflect.TypeOf((*MockStore)(nil).PaginatedOrganizationMembers), ctx, arg)
 }
 
+// PinChatByID mocks base method.
+func (m *MockStore) PinChatByID(ctx context.Context, id uuid.UUID) error {
+	m.ctrl.T.Helper()
+	ret := m.ctrl.Call(m, "PinChatByID", ctx, id)
+	ret0, _ := ret[0].(error)
+	return ret0
+}
+
+// PinChatByID indicates an expected call of PinChatByID.
+func (mr *MockStoreMockRecorder) PinChatByID(ctx, id any) *gomock.Call {
+	mr.mock.ctrl.T.Helper()
+	return mr.mock.ctrl.RecordCallWithMethodType(mr.mock, "PinChatByID", reflect.TypeOf((*MockStore)(nil).PinChatByID), ctx, id)
+}
+
 // Ping mocks base method.
 func (m *MockStore) Ping(ctx context.Context) (time.Duration, error) {
 	m.ctrl.T.Helper()
@@ -7614,6 +7628,20 @@ func (mr *MockStoreMockRecorder) UnfavoriteWorkspace(ctx, id any) *gomock.Call {
 	return mr.mock.ctrl.RecordCallWithMethodType(mr.mock, "UnfavoriteWorkspace", reflect.TypeOf((*MockStore)(nil).UnfavoriteWorkspace), ctx, id)
 }
 
+// UnpinChatByID mocks base method.
+func (m *MockStore) UnpinChatByID(ctx context.Context, id uuid.UUID) error {
+	m.ctrl.T.Helper()
+	ret := m.ctrl.Call(m, "UnpinChatByID", ctx, id)
+	ret0, _ := ret[0].(error)
+	return ret0
+}
+
+// UnpinChatByID indicates an expected call of UnpinChatByID.
+func (mr *MockStoreMockRecorder) UnpinChatByID(ctx, id any) *gomock.Call {
+	mr.mock.ctrl.T.Helper()
+	return mr.mock.ctrl.RecordCallWithMethodType(mr.mock, "UnpinChatByID", reflect.TypeOf((*MockStore)(nil).UnpinChatByID), ctx, id)
+}
+
 // UnsetDefaultChatModelConfigs mocks base method.
 func (m *MockStore) UnsetDefaultChatModelConfigs(ctx context.Context) error {
 	m.ctrl.T.Helper()
@@ -7762,6 +7790,20 @@ func (mr *MockStoreMockRecorder) UpdateChatModelConfig(ctx, arg any) *gomock.Cal
 	return mr.mock.ctrl.RecordCallWithMethodType(mr.mock, "UpdateChatModelConfig", reflect.TypeOf((*MockStore)(nil).UpdateChatModelConfig), ctx, arg)
 }
 
+// UpdateChatPinOrder mocks base method.
+func (m *MockStore) UpdateChatPinOrder(ctx context.Context, arg database.UpdateChatPinOrderParams) error {
+	m.ctrl.T.Helper()
+	ret := m.ctrl.Call(m, "UpdateChatPinOrder", ctx, arg)
+	ret0, _ := ret[0].(error)
+	return ret0
+}
+
+// UpdateChatPinOrder indicates an expected call of UpdateChatPinOrder.
+func (mr *MockStoreMockRecorder) UpdateChatPinOrder(ctx, arg any) *gomock.Call {
+	mr.mock.ctrl.T.Helper()
+	return mr.mock.ctrl.RecordCallWithMethodType(mr.mock, "UpdateChatPinOrder", reflect.TypeOf((*MockStore)(nil).UpdateChatPinOrder), ctx, arg)
+}
+
 // UpdateChatProvider mocks base method.
 func (m *MockStore) UpdateChatProvider(ctx context.Context, arg database.UpdateChatProviderParams) (database.ChatProvider, error) {
 	m.ctrl.T.Helper()