feat(themes): Add AE_Firefly variant themes — SteelBlue, Indigo, Rainbow

Three new Firefly-family themes following the AE_Firefly design system:
- AE_Firefly_SteelBlue: metallic steel blue primary (~214°), burnished gold
  secondary, cobalt navy tertiary, chrome silver surfaces
- AE_Firefly_Indigo: deep indigo primary (~266°), violet secondary, dusty
  rose tertiary, velvet slate surfaces
- AE_Firefly_Rainbow: coral-red primary (~15°), emerald green secondary,
  rich violet tertiary, sunrise cream surfaces (spans the visible spectrum)

All variants share consistent semantic colors (success/warning/error) with
AE_Firefly for cross-theme recognizability. All WCAG 2.1 AA compliant.

Also adds URL param support for theme switching:
- ?theme=AE_Firefly_SteelBlue&theme_mode=dark
- Params applied to ae_loc (persisted), then silently removed via replaceState

src/routes/+layout.svelte

@@ -234,6 +234,23 @@
         if (iframe === 'true') $ae_loc.iframe = true;
         else if (iframe === 'false') $ae_loc.iframe = false;
 
+        // Theme URL params — ?theme=AE_Firefly_SteelBlue&theme_mode=dark
+        // Applied once on load, then silently removed from the URL (no history entry).
+        const url_theme = data.url.searchParams.get('theme');
+        const url_theme_mode = data.url.searchParams.get('theme_mode');
+        if (url_theme || url_theme_mode) {
+            if (url_theme) $ae_loc.theme_name = url_theme;
+            if (url_theme_mode === 'light' || url_theme_mode === 'dark') $ae_loc.theme_mode = url_theme_mode;
+            const clean_url = new URL(data.url.href);
+            clean_url.searchParams.delete('theme');
+            clean_url.searchParams.delete('theme_mode');
+            goto(clean_url.pathname + clean_url.search + clean_url.hash, {
+                replaceState: true,
+                noScroll: true,
+                keepFocus: true
+            });
+        }
+
         // Electron Detection
         if ((window as any).native_app || (window as any).aetherNative) {
             if (!$ae_loc.is_native) $ae_loc.is_native = true;