Items 16-22: Icon classification, accessibility, responsive design, API params

- Consolidate 5 weather icon methods into classifyWeatherIcon/applyWeatherIconClasses
- Add focus-visible styles, ARIA attributes, keyboard nav on config button/modal
- Add responsive breakpoints for departure cards, weather widget, config modal
- Simplify CSS selectors: replace :not(:is(...)) chains with body.normal
- Fix upsidedown layout margin assumption with transform-based centering
- Upgrade weather icons from @2x to @4x for high-DPI displays
- Add forecast window and transport filter params to SL departures API

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

index.html

@@ -133,7 +133,7 @@
         <div class="config-modal-content">
             <div class="config-modal-header">
                 <h2>Settings</h2>
-                <span class="config-modal-close">&times;</span>
+                <span class="config-modal-close" role="button" tabindex="0" aria-label="Close settings">&times;</span>
             </div>
             <div class="config-tabs">
                 <button class="config-tab active" data-tab="display">Display</button>