Make app permanently light mode

[devin-ai-integration]

Summary

Enforces permanent light mode by removing all dark mode functionality. The app will always render with the light theme — the Dark Mode toggle has been removed from settings, dark theme CSS variables have been deleted, and the data-theme attribute is always cleared on mount.

Changes:

• src/App.jsx: Removed isDarkMode state, theme localStorage, handleDarkMode handler, and dark theme effect logic
• src/components/SettingModal/SettingModal.jsx: Removed Dark Mode toggle row and related props
• src/App.module.scss: Removed [data-theme='dark'] CSS variable block
• public/index.html & public/manifest.json: Updated theme-color from #000000 to #ffffff

Screenshot

Review & Testing Checklist for Human

• Verify the app renders in light mode on first load (white background, dark text)
• Open Settings modal and confirm Dark Mode toggle is gone; Hard Mode and High Contrast toggles still work
• Verify high contrast mode still toggles correctly

Notes

• No CI is configured on this repo.
• The test suite has a pre-existing failure on the base branch (useAlert context issue) unrelated to these changes.

Link to Devin session: https://app.devin.ai/sessions/574999d864e04588a9c259b463d0e52a
Requested by: @shahmir-masood

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