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/querier.go

@@ -809,6 +809,12 @@ type sqlcQuerier interface {
 	//  - Use both to get a specific org member row
 	OrganizationMembers(ctx context.Context, arg OrganizationMembersParams) ([]OrganizationMembersRow, error)
 	PaginatedOrganizationMembers(ctx context.Context, arg PaginatedOrganizationMembersParams) ([]PaginatedOrganizationMembersRow, error)
+	// Under READ COMMITTED, concurrent pin operations for the same
+	// owner may momentarily produce duplicate pin_order values because
+	// each CTE snapshot does not see the other's writes. The next
+	// pin/unpin/reorder operation's ROW_NUMBER() self-heals the
+	// sequence, so this is acceptable.
+	PinChatByID(ctx context.Context, id uuid.UUID) error
 	PopNextQueuedMessage(ctx context.Context, chatID uuid.UUID) (ChatQueuedMessage, error)
 	ReduceWorkspaceAgentShareLevelToAuthenticatedByTemplate(ctx context.Context, templateID uuid.UUID) error
 	RegisterWorkspaceProxy(ctx context.Context, arg RegisterWorkspaceProxyParams) (WorkspaceProxy, error)
@@ -836,6 +842,7 @@ type sqlcQuerier interface {
 	// This will always work regardless of the current state of the template version.
 	UnarchiveTemplateVersion(ctx context.Context, arg UnarchiveTemplateVersionParams) error
 	UnfavoriteWorkspace(ctx context.Context, id uuid.UUID) error
+	UnpinChatByID(ctx context.Context, id uuid.UUID) error
 	UnsetDefaultChatModelConfigs(ctx context.Context) error
 	UpdateAIBridgeInterceptionEnded(ctx context.Context, arg UpdateAIBridgeInterceptionEndedParams) (AIBridgeInterception, error)
 	UpdateAPIKeyByID(ctx context.Context, arg UpdateAPIKeyByIDParams) error
@@ -848,6 +855,7 @@ type sqlcQuerier interface {
 	UpdateChatMCPServerIDs(ctx context.Context, arg UpdateChatMCPServerIDsParams) (Chat, error)
 	UpdateChatMessageByID(ctx context.Context, arg UpdateChatMessageByIDParams) (ChatMessage, error)
 	UpdateChatModelConfig(ctx context.Context, arg UpdateChatModelConfigParams) (ChatModelConfig, error)
+	UpdateChatPinOrder(ctx context.Context, arg UpdateChatPinOrderParams) error
 	UpdateChatProvider(ctx context.Context, arg UpdateChatProviderParams) (ChatProvider, error)
 	UpdateChatStatus(ctx context.Context, arg UpdateChatStatusParams) (Chat, error)
 	UpdateChatWorkspaceBinding(ctx context.Context, arg UpdateChatWorkspaceBindingParams) (Chat, error)