Add light variants for red, blue, yellow, pink, orange colour schemes

Extends ticket 048's light_green pattern (light-inverted surface/border/text
scale + light's neutral danger colours, same base and accent hues as the
dark-surfaced counterpart) to the remaining five colour schemes across all
six files that must stay in sync: index.css, themes.ts, api.ts, settings.rs,
useSettingsPanel.ts, and NavMenu.tsx.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01XFUNZwCvz3kcod3352ZR2o

webapp/src/components/NavMenu.tsx

@@ -28,6 +28,11 @@ const COLOR_SCHEMES: ColorScheme[] = [
   'pink',
   'orange',
   'light_green',
+  'light_red',
+  'light_blue',
+  'light_yellow',
+  'light_pink',
+  'light_orange',
 ]
 
 /** Display label for a `ColorScheme`: underscores become spaces so `capitalize` (which titles each word) renders e.g. "light_green" as "Light Green". */