fix(js): resolve Biome lint warnings across 9 JS files

noUnusedVariables (catch bindings → optional catch syntax):
- app.js, file-upload.js, timezone-selector.js: } catch (e) { → } catch {
  ES2019 optional catch binding; e was unused in all three handlers

noUnusedVariables (dead assignments):
- app.js: remove const data= in display SSE stub (handler does nothing yet)
- api_client.js: remove const timeoutId= (setTimeout ID never used to cancel)
- custom-feeds.js: remove const oldIndex= (getAttribute result never read)
- schedule-picker.js: remove const compactMode= (never used in HTML build)
- select-dropdown.js: remove const icons= (icons not yet rendered in options)

noPrototypeBuiltins:
- day-selector.js: DAY_LABELS.hasOwnProperty(x) →
  Object.prototype.hasOwnProperty.call(DAY_LABELS, x)
  Safe form that works even on null-prototype objects

useIterableCallbackReturn:
- file-upload.js, notification.js: forEach(x => expr) →
  forEach(x => { expr; }) — forEach ignores return values;
  implicit return from arrow body was misleading

htmx-sse.js is a vendor extension file with old-style var/== patterns
that are correct for it; 18 Biome issues suppressed via Codacy API
rather than modifying the vendor source.

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

web_interface/static/v3/js/widgets/file-upload.js

@@ -294,7 +294,7 @@
         if (fileType !== 'json') {
             formData.append('plugin_id', pluginId);
         }
-        validFiles.forEach(file => formData.append('files', file));
+        validFiles.forEach(file => { formData.append('files', file); });
         
         try {
             const response = await fetch(customUploadEndpoint, {
@@ -741,7 +741,7 @@
         try {
             const date = new Date(dateString);
             return date.toLocaleDateString() + ' ' + date.toLocaleTimeString([], { hour: '2-digit', minute: '2-digit' });
-        } catch (e) {
+        } catch {
             return dateString;
         }
     };