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/dbmetrics/querymetrics.go

@@ -3920,6 +3920,14 @@ func (m queryMetricsStore) PaginatedOrganizationMembers(ctx context.Context, arg
 	return r0, r1
 }
 
+func (m queryMetricsStore) PinChatByID(ctx context.Context, id uuid.UUID) error {
+	start := time.Now()
+	r0 := m.s.PinChatByID(ctx, id)
+	m.queryLatencies.WithLabelValues("PinChatByID").Observe(time.Since(start).Seconds())
+	m.queryCounts.WithLabelValues(httpmw.ExtractHTTPRoute(ctx), httpmw.ExtractHTTPMethod(ctx), "PinChatByID").Inc()
+	return r0
+}
+
 func (m queryMetricsStore) PopNextQueuedMessage(ctx context.Context, chatID uuid.UUID) (database.ChatQueuedMessage, error) {
 	start := time.Now()
 	r0, r1 := m.s.PopNextQueuedMessage(ctx, chatID)
@@ -4024,6 +4032,14 @@ func (m queryMetricsStore) UnfavoriteWorkspace(ctx context.Context, id uuid.UUID
 	return r0
 }
 
+func (m queryMetricsStore) UnpinChatByID(ctx context.Context, id uuid.UUID) error {
+	start := time.Now()
+	r0 := m.s.UnpinChatByID(ctx, id)
+	m.queryLatencies.WithLabelValues("UnpinChatByID").Observe(time.Since(start).Seconds())
+	m.queryCounts.WithLabelValues(httpmw.ExtractHTTPRoute(ctx), httpmw.ExtractHTTPMethod(ctx), "UnpinChatByID").Inc()
+	return r0
+}
+
 func (m queryMetricsStore) UnsetDefaultChatModelConfigs(ctx context.Context) error {
 	start := time.Now()
 	r0 := m.s.UnsetDefaultChatModelConfigs(ctx)
@@ -4104,6 +4120,14 @@ func (m queryMetricsStore) UpdateChatModelConfig(ctx context.Context, arg databa
 	return r0, r1
 }
 
+func (m queryMetricsStore) UpdateChatPinOrder(ctx context.Context, arg database.UpdateChatPinOrderParams) error {
+	start := time.Now()
+	r0 := m.s.UpdateChatPinOrder(ctx, arg)
+	m.queryLatencies.WithLabelValues("UpdateChatPinOrder").Observe(time.Since(start).Seconds())
+	m.queryCounts.WithLabelValues(httpmw.ExtractHTTPRoute(ctx), httpmw.ExtractHTTPMethod(ctx), "UpdateChatPinOrder").Inc()
+	return r0
+}
+
 func (m queryMetricsStore) UpdateChatProvider(ctx context.Context, arg database.UpdateChatProviderParams) (database.ChatProvider, error) {
 	start := time.Now()
 	r0, r1 := m.s.UpdateChatProvider(ctx, arg)