fix: Resolve light/dark theme issues across dashboards and settings

- Add missing setThemePreference() and setAnimationsEnabled() functions
  to settings-manager.js; sync theme/animations dropdowns in _updateUI
- Fix base.html toggleTheme() saving to wrong localStorage key ('theme'
  instead of 'intercept-theme'), causing theme not to persist in ADS-B
  and AIS dashboards; also sync button icon and persist to server
- Add [data-theme="light"] CSS variable overrides to adsb_dashboard.css
  and ais_dashboard.css so the dashboards respond to light theme
- Fix GPS sky view canvas (gps.js) to read grid/label colours from CSS
  variables instead of hardcoded dark hex values; add MutationObserver
  to redraw immediately on theme change
- Fix satellite_dashboard.html polar plot functions to read background,
  accent and text colours from CSS variables

Closes #139

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

templates/layout/base.html

@@ -133,7 +133,17 @@
             const currentTheme = html.getAttribute('data-theme') || 'dark';
             const newTheme = currentTheme === 'dark' ? 'light' : 'dark';
             html.setAttribute('data-theme', newTheme);
-            localStorage.setItem('theme', newTheme);
+            localStorage.setItem('intercept-theme', newTheme);
+
+            const btn = document.getElementById('themeToggle');
+            if (btn) btn.textContent = newTheme === 'light' ? '🌙' : '☀️';
+
+            // Persist to server
+            fetch('/settings', {
+                method: 'POST',
+                headers: { 'Content-Type': 'application/json' },
+                body: JSON.stringify({ theme: newTheme })
+            }).catch(() => {});
         }
 
         // Apply saved theme