Remove duplicate "grey" theme, keep "dark" (ticket 004)

"grey" and "dark" rendered identically apart from accent colour, so
drop "grey" as a selectable colour scheme everywhere in the frontend
(index.css, api.ts's ColorScheme type, the NavMenu picker, the
downloadable BUILTIN_THEMES table) and keep "dark" as the single
equivalent theme.

Existing settings.json files saved with color_scheme: "grey" keep
loading correctly: the backend ColorScheme enum aliases the legacy
"grey" serde value onto Dark, so old configs transparently migrate to
"dark" without any manual intervention.

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

webapp/src/lib/themes.ts

@@ -72,28 +72,6 @@ export const BUILTIN_THEMES: Record<string, ThemeVars> = {
     '--c-accent-soft': '#7dd3fc',
     '--c-accent-text': '#0369a1',
   },
-  grey: {
-    '--c-surface-0': '#0a0a0a',
-    '--c-surface-1': '#171717',
-    '--c-surface-2': '#262626',
-    '--c-border': '#262626',
-    '--c-border-soft': '#404040',
-    '--c-border-faint': '#171717',
-    '--c-border-strong': '#a3a3a3',
-    '--c-text': '#f5f5f5',
-    '--c-text-secondary': '#d4d4d4',
-    '--c-text-muted': '#a3a3a3',
-    '--c-text-subtle': '#737373',
-    '--c-text-faint': '#525252',
-    '--c-text-faintest': '#404040',
-    '--c-danger-bg': '#450a0a',
-    '--c-danger-border': '#7f1d1d',
-    '--c-danger-text': '#f87171',
-    '--c-danger-text-strong': '#fca5a5',
-    '--c-accent': '#38bdf8',
-    '--c-accent-soft': '#0369a1',
-    '--c-accent-text': '#7dd3fc',
-  },
   red: {
     '--c-surface-0': 'hsl(0 32% 10%)',
     '--c-surface-1': 'hsl(0 28% 16%)',