feat(appearance): Sahara theme

Built from the three sands an operator sent: #faf6eb for panels, #e8dfca
for the page behind them, #ddd2ba for toolbars, table headers and rows.
The ordering is what makes it work — a panel reads as lifted off the page
and the log grid as settled into it, so rows stay scannable without extra
rules. Ink is a deep warm brown; pure black on sand glares.

frontend/src/lib/theme.tsx

@@ -6,14 +6,14 @@ import { GetUIPref } from '../../wailsjs/go/main/App';
 // CSS variables in style.css key off of. 'auto' follows the OS light/dark
 // preference. The choice is persisted (localStorage + portable UI pref, so it
 // travels with the data/ folder like the language).
-export type ThemeChoice = 'auto' | 'light-warm' | 'light-cool' | 'light-sage' | 'light-nordic'
+export type ThemeChoice = 'auto' | 'light-warm' | 'light-cool' | 'light-sage' | 'light-nordic' | 'sahara'
   | 'dim-slate' | 'dark-warm' | 'dark-graphite' | 'dark-indigo' | 'dark-teal' | 'dark-plum' | 'high-contrast';
 
 // Selectable, concrete themes (excludes 'auto') in display order: lights first,
 // then darks, with high-contrast last — it is an accessibility choice, not a
 // taste one, and listing it among the moods buries it.
 export const CONCRETE_THEMES: Exclude<ThemeChoice, 'auto'>[] = [
-  'light-warm', 'light-cool', 'light-sage', 'light-nordic',
+  'light-warm', 'light-cool', 'light-sage', 'light-nordic', 'sahara',
   'dim-slate', 'dark-warm', 'dark-graphite', 'dark-indigo', 'dark-teal', 'dark-plum',
   'high-contrast',
 ];