feat(web): add theme system with CSS variables and settings modal

[theonlyhennygod]

Summary

• Adds ThemeContext, ThemeProvider, themeStorage, and useTheme hook for managing theme mode, accent color, UI/mono fonts and font sizes with localStorage persistence
• Creates SettingsModal component for live preview of theme, accent, font, and size choices
• Integrates ThemeProvider into App, adds settings button in Header
• Updates Layout/Sidebar/ErrorBoundary/PairingDialog and all page components to use CSS custom properties instead of hardcoded colors
• Simplifies locale/context typings

Supersedes

• feat(web): add theme system with CSS variables and settings modal #4122 by @nanyuantingfeng

Integrated Scope

• From feat(web): add theme system with CSS variables and settings modal #4122: full implementation cherry-picked — theme context, settings modal, CSS variable migration, i18n keys

Attribution

• Co-authored-by trailer added for materially incorporated contributor: Yes

Non-goals

• No backend changes — web-only
• No new dependencies added

Risk and Rollback

• Risk: Low — web UI only, no backend/security impact
• Rollback: revert commit

Test plan

• Verify theme switching (system/dark/light/oled) applies correctly
• Verify accent color changes reflect across all components
• Verify font and size settings persist across page reloads
• Verify settings modal opens/closes properly
• Verify existing pages render correctly with new CSS variables

Co-authored-by: nanyuantingfeng nanyuantingfeng@163.com

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