feat(auth): redesign auth UI to match Dart original

- Full-bleed bg_login.png background outside 500px MobileShell
- Facebook landing: logo + Facebook pill button + "Other Sign In Options" link
- Replace centered Dialog with true bottom sheet (drag handle, top radius 28px)
- Email panel: internal login/register toggle, no separate route
- Pink gradient primary button (#f96ADE → #f657A0)
- Pill input fields (46px, white, radius 24, pink 10% shadow)
- Pink circular checkbox + bold Privacy/Terms rich text (visual only, non-blocking)
- Floating 40x40 white back button (top:20/left:20)
- Wire AuthPanelModeChanged dispatch for panel switching
- New design tokens (--color-auth-*, --auth-*, --radius-bottom-sheet)
- New generic BottomSheet component
- Copy ic-logo-login.png (+@2x, +@3x) from Dart assets

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

src/app/auth/components/auth-primary-button.module.css

@@ -1,22 +1,37 @@
+/* 原始 Dart: lib/ui/auth/widgets/auth_primary_button.dart
+ *
+ * 视觉规格（与 Dart 对齐）：
+ *   - 46px 高
+ *   - 粉渐变（#f96ADE → #f657A0，左→右）
+ *   - 半径 24
+ *   - 20% 红阴影（#d00c41 @ 20%）
+ *   - 文字 16px bold 白色
+ */
 .button {
   display: inline-flex;
   align-items: center;
   justify-content: center;
   gap: var(--spacing-sm);
   width: 100%;
-  height: var(--button-height);
+  height: var(--auth-field-height);
   padding: 0 var(--spacing-lg);
   border: none;
-  border-radius: var(--radius-full);
-  font-size: var(--font-size-md);
-  font-weight: 600;
-  color: var(--color-text-primary);
+  border-radius: var(--radius-xxxl);
+  background: linear-gradient(
+    90deg,
+    var(--color-auth-primary-gradient-start),
+    var(--color-auth-primary-gradient-end)
+  );
+  box-shadow: 0 3px 5px var(--color-auth-primary-button-shadow);
+  color: #ffffff;
+  font-size: var(--font-size-lg);
+  font-weight: 700;
   cursor: pointer;
-  transition: opacity 0.15s ease, transform 0.05s ease;
+  transition: filter 0.15s ease, transform 0.05s ease, opacity 0.15s ease;
 }
 
 .button:disabled {
-  opacity: 0.6;
+  opacity: 0.7;
   cursor: not-allowed;
 }
 
@@ -24,25 +39,18 @@
   transform: scale(0.98);
 }
 
-.accent {
-  background: linear-gradient(
-    90deg,
-    var(--color-accent),
-    var(--color-facebook-button-gradient-end)
-  );
-}
-
-.primary {
-  background: var(--color-primary);
+.label {
+  flex: 1 1 auto;
+  text-align: center;
 }
 
 .spinner {
   display: inline-block;
-  width: 16px;
-  height: 16px;
+  width: 18px;
+  height: 18px;
   border-width: 2px;
   border-style: solid;
-  border-color: currentColor transparent transparent transparent;
+  border-color: #ffffff transparent transparent transparent;
   border-radius: 50%;
   animation: spin 0.8s linear infinite;
 }