Allow shared search links to work without login

- Remove auth from poster proxy (artwork isn't sensitive, API key
  stays server-side)
- Show main screen in read-only mode when ?s= param is present,
  hiding user picker, logout, and re-roll controls
- If viewer happens to be logged in, watch-check still runs

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

app/main.py

@@ -2,7 +2,7 @@ import asyncio
 import logging
 from contextlib import asynccontextmanager
 
-from fastapi import FastAPI, Request
+from fastapi import FastAPI
 from fastapi.responses import Response as FastAPIResponse
 from fastapi.staticfiles import StaticFiles
 
@@ -41,10 +41,7 @@ except ImportError:
 
 
 @app.get("/api/poster/{item_id}")
-async def poster_proxy(item_id: str, request: Request):
-    from app.routers.auth import get_current_user
-    await get_current_user(request)
-
+async def poster_proxy(item_id: str):
     image_data = await get_poster(item_id)
     if image_data is None:
         return FastAPIResponse(status_code=404)