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/components/SettingsModal.tsx

@@ -412,6 +412,7 @@ const THEME_SWATCH: Record<Exclude<ThemeChoice, 'auto'>, { bg: string; card: str
   'light-warm':    { bg: '#e8dfc9', card: '#faf6ea', accent: '#b8410c' },
   'light-cool':    { bg: '#f4f6f8', card: '#ffffff', accent: '#2563eb' },
   'light-sage':    { bg: '#eef1ec', card: '#f8faf6', accent: '#2f855a' },
+  'sahara':        { bg: '#e8dfca', card: '#faf6eb', accent: '#b45309' },
   'light-nordic':  { bg: '#eef1f7', card: '#ffffff', accent: '#4f46e5' },
   'dim-slate':     { bg: '#343b47', card: '#3d4552', accent: '#fb923c' },
   'dark-warm':     { bg: '#221d18', card: '#2e2820', accent: '#e07a2e' },