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/src/styles.css

@@ -1,3 +1,32 @@
+:root {
+  --bg:             #0f0f0f;
+  --surface:        #1a1a1a;
+  --surface-raised: #252525;
+  --border:         #333;
+  --border-focus:   #555;
+  --text:           #e0e0e0;
+  --text-muted:     #777;
+  --accent:         #4a9eff;
+  --accent-text:    #000;
+  --danger:         #c0392b;
+  --danger-text:    #fff;
+  --radius:         6px;
+  --radius-chip:    12px;
+  --transition:     200ms ease;
+}
+
+@keyframes shimmer {
+  from { background-position: -200% 0; }
+  to   { background-position:  200% 0; }
+}
+
+.skeleton {
+  background: linear-gradient(90deg, var(--surface) 25%, var(--surface-raised) 50%, var(--surface) 75%);
+  background-size: 200% 100%;
+  animation: shimmer 1.4s infinite;
+  border-radius: var(--radius);
+}
+
 * {
   box-sizing: border-box;
   margin: 0;
@@ -6,7 +35,7 @@
 
 body {
   font-family: system-ui, -apple-system, sans-serif;
-  background: #0f0f0f;
-  color: #e0e0e0;
+  background: var(--bg);
+  color: var(--text);
   min-height: 100vh;
 }