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.

web/templates/settings_picture.html

@@ -6,17 +6,15 @@
 <div class="toast error">{{ error }}</div>
 {% endif %}
 
-{% if settings.mode == "upload" and settings.uploaded_filename %}
+{% if settings.uploaded_filename %}
 <div class="card">
   <div class="section-label">Current picture</div>
   <div class="preview-wrap">
     <img class="preview" src="{{ url_for('static', filename='uploads/' + settings.uploaded_filename) }}" alt="current picture">
   </div>
-</div>
-{% elif settings.mode == "url" and settings.image_url %}
-<div class="card">
-  <div class="section-label">Current picture</div>
-  <p class="hint">Fetched from a URL: {{ settings.image_url }}</p>
+  {% if settings.mode == "url" and settings.image_url %}
+  <p class="hint">Downloaded from: {{ settings.image_url }}</p>
+  {% endif %}
 </div>
 {% endif %}
 
@@ -35,7 +33,7 @@
     <div class="field">
       <label for="image_url">Image URL</label>
       <input type="text" id="image_url" name="image_url" placeholder="https://example.com/photo.jpg">
-      <p class="hint">Re-fetched every 30 minutes. Leave both this and the upload above alone to just change the options below.</p>
+      <p class="hint">Downloaded once and cached -- won't keep re-fetching, so this can't cause the panel to flash on its own. Enter a new URL any time to replace it. Leave both this and the upload above alone to just change the options below.</p>
     </div>
   </div>