Feat: Polish Angular UI with cohesive design system

Introduces a shared CSS custom property token layer and applies it
across all five views (library, upload, detail, login, app shell).
Each view now has intentional loading, empty, and error states.

- styles.css: 13 design tokens on :root; shimmer skeleton animation
- Library: 150ms-debounced skeleton loading, empty state with /upload
  link, error card with retry, card hover lift, broken-image fallback
- Upload: token-styled drop-zone, Uploading… spinner, 4s success
  banner, distinct validation vs. network error messages
- Detail: image skeleton, network error card (separate from 404
  not-found card), Owner actions panel, danger tag error styling,
  broken-image fallback
- Login: vertically centred surface card, danger field/server errors,
  Signing in… disabled button
- App shell: 48px fixed header, app name left, sign-out right, no
  reflow on auth state change
- All 24 ESLint errors resolved (including pre-existing auth spec
  issues); ng build and ng lint pass clean

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

ui/.prettierignore

@@ -0,0 +1,4 @@
+dist/
+node_modules/
+coverage/
+package-lock.json