feat(ui): add dark mode support with system preference and toggle (#209)

[vaquarkhan]

Phase 1: enable the dark mode mechanism and apply it to the app shell.

• tailwind.config.js: change darkMode from invalid 'false' to 'class'
• Add useTheme hook: respects system preference via prefers-color-scheme, allows manual override, persists choice to localStorage, reacts to OS changes
• Add ThemeToggle component (sun/moon button) replacing the previously broken radio
• Apply dark: variants to the app shell: container, desktop + mobile sidebar, nav links, disclosure menus, breadcrumb, and project list table
• Mount theme initialization at App root to apply the dark class on load

Light mode is unchanged (all changes are additive dark: variants). Remaining per-view component coverage will follow in subsequent PRs.

[Short description explaining the high-level reason for the pull request]

Changes

How I tested this

Notes

Checklist

• PR has an informative and human-readable title (this will be pulled into the release notes)
• Changes are limited to a single goal (no scope creep)
• Code passed the pre-commit check & code is left cleaner/nicer than when first encountered.
• Any change in functionality is tested
• New functions are documented (with a description, list of inputs, and expected output)
• Placeholder code is flagged / future TODOs are captured in comments
• Project documentation has been updated if adding/changing functionality.

[Viquarkhan-aws]

[Viquarkhan-aws]