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/custom-feeds.js

@@ -364,7 +364,6 @@
             // Re-index remaining rows
             const rows = tbody.querySelectorAll('.custom-feed-row');
             rows.forEach((r, index) => {
-                const oldIndex = r.getAttribute('data-index');
                 r.setAttribute('data-index', index);
                 // Update all input names with new index
                 r.querySelectorAll('input, button').forEach(input => {