feature: add explicit light mode theme (default)

[devin-ai-integration]

Summary

Makes light mode an explicit, well-defined theme and the default on first load. Previously "off" dark mode just removed data-theme, relying on the :root fallback; now light mode is a named theme set via data-theme="light", mirroring how dark works.

Key changes:

• App.module.scss: added an explicit [data-theme='light'] variable block (alongside the existing :root defaults and [data-theme='dark']), and changed the hardcoded body { color: #fff } to var(--color-text-primary) so text is readable in light mode.
• App.jsx: the theme effect now sets the attribute in both directions instead of removing it:

  - if (isDarkMode) document.body.setAttribute('data-theme', 'dark');
  - else document.body.removeAttribute('data-theme');
  + if (isDarkMode) document.body.setAttribute('data-theme', 'dark');
  + else document.body.setAttribute('data-theme', 'light');

• App.jsx: default stored theme changed from 'dark' to 'light', so first-time visitors get light mode. The existing toggle in SettingModal ("Dark Mode") already persists 'light'/'dark' to localStorage via handleDarkMode, so the choice continues to survive reloads.

Screenshots

Light mode (new default):

Dark mode (via Settings → Dark Mode, persists across reloads):

Testing

• npm run check (prettier) passes.
• Verified in browser: light default on fresh load, toggling Dark Mode switches data-theme and persists (localStorage.theme="dark", body[data-theme="dark"]) across reload.
• Note: the single existing Jest test (App.test.jsx) fails on main as well (unrelated — useAlert is undefined without its provider in the test). Not introduced by this PR.

Link to Devin session: https://app.devin.ai/sessions/edc13b821f804c1ebec949e766235288
Requested by: @shahmir-masood

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