Add minimal installable PWA support

- serve manifest and service worker from the app root for install compatibility
- add manifest metadata, service worker registration, and Apple touch icon links to the base template
- add install icon assets for Android, iOS, and desktop install flows
- document deployment and validation notes for the new PWA support
- cover the new endpoints and template output with tests

app/main.py

@@ -1,7 +1,8 @@
 from contextlib import asynccontextmanager
+from pathlib import Path
 
 from fastapi import Depends, FastAPI, File, Form, HTTPException, Request, UploadFile, status
-from fastapi.responses import RedirectResponse, Response
+from fastapi.responses import FileResponse, RedirectResponse, Response
 from fastapi.staticfiles import StaticFiles
 from fastapi.templating import Jinja2Templates
 from sqlalchemy import func, or_
@@ -12,6 +13,7 @@ from app.images import process_upload
 from app.models import Box, Item, SubItem
 
 templates = Jinja2Templates(directory="app/templates")
+STATIC_DIR = Path("app/static")
 
 
 def _clean_text(value: str | None) -> str | None:
@@ -193,6 +195,20 @@ def create_app() -> FastAPI:
     def root() -> RedirectResponse:
         return RedirectResponse(url="/boxes", status_code=status.HTTP_302_FOUND)
 
+    @app.get("/manifest.webmanifest", include_in_schema=False)
+    def manifest() -> FileResponse:
+        return FileResponse(
+            path=STATIC_DIR / "manifest.webmanifest",
+            media_type="application/manifest+json",
+        )
+
+    @app.get("/service-worker.js", include_in_schema=False)
+    def service_worker() -> FileResponse:
+        return FileResponse(
+            path=STATIC_DIR / "service-worker.js",
+            media_type="application/javascript",
+        )
+
     @app.get("/search")
     def search_page(
         request: Request,