feature: add Light Mode Only mode

[devin-ai-integration]

Summary

Adds a Light Mode Only setting that locks the app to the light theme and disables the Dark Mode toggle. Useful for users who never want the app to render in dark mode (e.g. shared/kiosk setups). The preference persists via localStorage under light-mode-only.

When enabled it forces the theme to light and disables the Dark Mode row so it can't be turned back on:

const handleLightModeOnly = () => {
  const next = !isLightModeOnly;
  setIsLightModeOnly(next);
  setLightModeOnly(next);        // persisted to localStorage
  if (next) {
    setIsDarkMode(false);
    setTheme('light');
  }
};

// theme effect only applies dark when not locked
if (isDarkMode && !isLightModeOnly) body.setAttribute('data-theme', 'dark');
else body.removeAttribute('data-theme');

The Switch component gained a disabled prop (native disabled + dimmed .disabled style + click guard), used to lock the Dark Mode row while Light Mode Only is active.

Files

• src/App.jsx: new light-mode-only localStorage state, handleLightModeOnly, dark-mode guard, props to SettingModal.
• src/components/SettingModal/SettingModal.jsx: new "Light Mode Only" row; Dark Mode row disabled when active.
• src/components/Switch/Switch.jsx + .module.scss: disabled prop support and styling.

Verification

Ran npm run check (Prettier) and npm run build — both pass. Manually tested in the browser: enabling Light Mode Only switches the UI to light and locks the Dark Mode toggle (clicking it does nothing).

Link to Devin session: https://app.devin.ai/sessions/09f6572e8db04aeeb65c2b28bafdf34d
Requested by: @shahmir-masood

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