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/lib/useSettingsPanel.ts

@@ -31,6 +31,11 @@ const THEME_CLASSES = [
   'pink',
   'orange',
   'light_green',
+  'light_red',
+  'light_blue',
+  'light_yellow',
+  'light_pink',
+  'light_orange',
 ] as const
 
 // Reflects `colorScheme` onto `<html>` as a class + `data-color-scheme`; when a custom theme is active, no built-in class is added at all, since a class selector would out-specificity its injected `:root` override (see `applyCustomThemeCss`) and mask it.