Add persistent theme toggle

[MFA-G]

Summary

• add a light/dark mode toggle that stores the selected preference in localStorage
• initialize the UI from the saved preference or the system color scheme
• replace the starter Vite screen with a responsive IssueFlow dashboard hero using theme-aware CSS variables

Closes #5

Verification

• corepack pnpm lint
• corepack pnpm build

Note: the production build still prints existing Lightning CSS warnings for the repository's @tailwind directives in src/index.css, but completes successfully.

[DavisVT]

Thanks @MFA-G for the solution