Feat: modern UI design & improvements

[oxdc]

I redesigned the frontend UI with a focus on minimal, professional style and better mobile support. This PR is meant to be technically lightweight. Most changes are CSS and small HTML tweaks, no architectural changes.

What changed:

• CSS rewrite: consistent design tokens, colors, spacing, typography, and light/dark mode throughout
• Mobile support: dialogs go full-screen on small viewports, buttons and forms wrap properly, no more overflowing tables
• Small HTML tweaks: label casing fixed (Display name, ID, Principal…), button hierarchy (neutral default, primary only where it matters, red-tinted danger), and a few layout cleanups
• Fixes frontend: Improve app token section #147 : token generation no longer redirects to a blank page; the token appears inline in an amber-highlighted area with a copy button and a "shown once" warning
• UX polish: clicking outside a modal closes it

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