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/api.ts

@@ -85,7 +85,6 @@ export type ColorScheme =
   | 'light'
   | 'dark'
   | 'system'
-  | 'grey'
   | 'red'
   | 'green'
   | 'blue'

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%)',

webapp/src/lib/useSettingsPanel.ts

@@ -24,7 +24,7 @@ const INJECTED_CSS_ATTR = 'data-ottyl-css-id'
 const INJECTED_THEME_ATTR = 'data-ottyl-theme-css-id'
 
 // Themes selectable independently of `system`, each backed by a `html.<name>` block in `index.css`.
-const THEME_CLASSES = ['light', 'dark', 'grey', 'red', 'green', 'blue', 'yellow', 'pink', 'orange'] as const
+const THEME_CLASSES = ['light', 'dark', 'red', 'green', 'blue', 'yellow', 'pink', '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.
 function applyColorScheme(colorScheme: ColorScheme, customThemeActive: boolean) {