Phase 14: multi-host file browser (0.15.0)

The Files page gained a host switcher: when agents are registered, a Host
dropdown switches the whole browser between the local host and any online agent
(switching resets path + clipboard). Every file operation is sandboxed by the
selected agent's own ALLOWED_BROWSE_ROOTS/HOST_ROOT_PREFIX.

- agent_app.py: /agent/files/* (list/read/download/write/mkdir/touch/rename/
  copy/move/delete/upload) reusing file_service + device_service; BrowseError
  -> HTTP 400.
- routers/agents.py: proxy routes at /api/agents/{id}/files/* (audit-logged
  mutations); download streams via download_to_file, upload via upload_file.
  Reuses the WriteBody/NameBody/RenameBody/TransferBody models from routers.files.
- Frontend: filesApi methods take an optional trailing agentId; Files.tsx tracks
  a host and threads it through every call, query key, and the editor/dialogs.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

backend/main.py

@@ -55,7 +55,7 @@ async def lifespan(app: FastAPI):
     schedule_task.cancel()
 
 
-app = FastAPI(title="StackPilot", version="0.14.0", lifespan=lifespan)
+app = FastAPI(title="StackPilot", version="0.15.0", lifespan=lifespan)
 
 app.add_middleware(
     CORSMiddleware,