Picture: download URLs once at save-time, not on every poll

Previously check() re-fetched the configured image URL over HTTP on
every poll (every 30 min by default). If the remote image ever came
back even slightly byte-different between fetches -- common with
dynamically-generated images -- that forced a real e-ink flash for no
visible reason, since this panel has no partial refresh.

Now the URL is downloaded exactly once, server-side in the settings
route, and saved into web/static/uploads/current.<ext> the same way a
direct upload already was -- clearing any previous upload/download
first so switching sources doesn't accumulate orphaned files.
check() just reads that local file regardless of whether it came from
an upload or a URL, with zero network calls. Also added a browser
User-Agent to the download request -- confirmed live that Wikimedia
(plausibly a common source people would link to) 403s the default
requests/python User-Agent as bot traffic, same class of issue as the
EFA transit endpoints handled earlier.

Verified end-to-end against real URLs: download+cache works, a second
URL replaces rather than accumulates, and a settings-only save (no
new URL entered) no longer wipes the cached picture.

screens/picture.py

@@ -1,13 +1,17 @@
-"""Shows a picture -- either uploaded through the web UI or fetched from a
-URL on a timer. Same contrast-boost/grayscale treatment as the Navidrome
-cover art, since the same e-ink legibility concerns apply.
+"""Shows a picture -- either uploaded through the web UI or downloaded from a
+URL. Either way it ends up as a local file in UPLOAD_DIR and check() just
+reads that file; a URL is downloaded once, at settings-save time (in
+web/app.py), not re-fetched on every poll -- this screen has no partial
+refresh any more than the rest of the panel does, so re-fetching on a timer
+would risk a pointless flash every poll_interval if the remote image ever
+came back byte-different despite looking the same. Same contrast-boost/
+grayscale treatment as the Navidrome cover art, since the same e-ink
+legibility concerns apply.
 """
 
 import logging
-from io import BytesIO
 from pathlib import Path
 
-import requests
 from PIL import Image, ImageDraw, ImageEnhance
 
 from screens.base import Screen
@@ -15,18 +19,17 @@ from screens.render_utils import fit_contain, fit_cover, load_font, text_width
 
 logger = logging.getLogger(__name__)
 
-HTTP_TIMEOUT = 15
 UPLOAD_DIR = Path(__file__).resolve().parent.parent / "web" / "static" / "uploads"
 
 
 class PictureScreen(Screen):
     id = "picture"
     display_name = "Picture"
-    poll_interval = 1800  # 30 minutes -- only matters for url mode
+    poll_interval = 1800  # just a cheap local file re-read, doesn't need to be frequent
 
     def default_settings(self) -> dict:
         return {
-            "mode": "url",  # "url" or "upload"
+            "mode": "url",  # "url" or "upload" -- purely informational (which source the current cached file came from), check() treats both the same
             "image_url": "",
             "uploaded_filename": "",
             "fit": "cover",  # "cover" or "contain"
@@ -35,30 +38,17 @@ class PictureScreen(Screen):
         }
 
     def check(self, settings: dict):
-        mode = settings.get("mode", "url")
-        if mode == "upload":
-            filename = settings.get("uploaded_filename")
-            if not filename:
-                return None
-            path = UPLOAD_DIR / filename
-            if not path.exists():
-                logger.warning("Uploaded picture %s is missing", path)
-                return None
-            try:
-                return Image.open(path).convert("RGB")
-            except OSError as exc:
-                logger.warning("Could not open uploaded picture %s: %s", path, exc)
-                return None
-
-        url = (settings.get("image_url") or "").strip()
-        if not url:
+        filename = settings.get("uploaded_filename")
+        if not filename:
+            return None
+        path = UPLOAD_DIR / filename
+        if not path.exists():
+            logger.warning("Picture %s is missing", path)
             return None
         try:
-            response = requests.get(url, timeout=HTTP_TIMEOUT)
-            response.raise_for_status()
-            return Image.open(BytesIO(response.content)).convert("RGB")
-        except (requests.exceptions.RequestException, OSError) as exc:
-            logger.warning("Could not load picture from %s: %s", url, exc)
+            return Image.open(path).convert("RGB")
+        except OSError as exc:
+            logger.warning("Could not open picture %s: %s", path, exc)
             return None
 
     def render(self, state, settings: dict, resolution: tuple[int, int]) -> Image.Image: