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>

src/fabledassistant/services/generation_buffer.py

@@ -37,6 +37,9 @@ class GenerationBuffer:
     finished_at: float | None = None
     cancel_event: asyncio.Event = field(default_factory=asyncio.Event)
     _notify: asyncio.Event = field(default_factory=asyncio.Event)
+    # Tool confirmation — set when a write tool is awaiting user approval
+    confirmation_future: asyncio.Future | None = None
+    pending_tool: dict | None = None
 
     def append_event(self, event_type: str, data: dict) -> SSEEvent:
         event = SSEEvent(index=len(self.events), event_type=event_type, data=data)