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/api.ts

@@ -91,6 +91,11 @@ export type ColorScheme =
   | 'pink'
   | 'orange'
   | 'light_green'
+  | 'light_red'
+  | 'light_blue'
+  | 'light_yellow'
+  | 'light_pink'
+  | 'light_orange'
 
 export interface Settings {
   color_scheme: ColorScheme