Add tool confirmation UI with Accept/Decline for write operations

Before executing any write tool (create/update/delete), the backend now
pauses with an asyncio.Future and emits a tool_pending SSE event. The
frontend displays a ToolConfirmCard with Accept and Decline buttons.
Clicking Accept resolves the Future and proceeds; Decline records a
declined tool_call chip and falls through to regular streaming. Typing
single-word yes/no responses (e.g. "yes", "cancel") also works as
confirmation. 120s timeout auto-declines if no response.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/src/types/chat.ts

@@ -10,7 +10,13 @@ export interface ToolCallRecord {
   function: string;
   arguments: Record<string, unknown>;
   result: { success: boolean; type?: string; data?: Record<string, unknown>; error?: string; suggested_tags?: string[] };
-  status: "success" | "error";
+  status: "success" | "error" | "declined";
+}
+
+export interface ToolPendingRecord {
+  function: string;
+  arguments: Record<string, unknown>;
+  label?: string;
 }
 
 export interface Message {