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/app.css

@@ -28,6 +28,9 @@
 @import './ae-c-lci.css';
 @import './ae-c-idaa-light.css';
 @import './ae-firefly.css';
+@import './ae-firefly-steelblue.css';
+@import './ae-firefly-indigo.css';
+@import './ae-firefly-rainbow.css';
 
 @source '../node_modules/@skeletonlabs/skeleton-svelte/dist';