style(sidebar): align /sidebar screen with Flutter original

Match the Dart lib/ui/sidebar/sidebar_screen.dart + profile_view.dart
exactly:
- White inner shell with 24px padding
- Back button row (arrow icon + "Back" text, no "Profile" header)
- Single "Other" section with one "Log out" row
- Drop profile card, Account/Data/Session sections, and Sign-in CTA

Match Flutter row visuals in the shared SettingsSection:
- Gray (#f5f5f5) rounded row pill on a white card
- 20px chevron, 1px rgba(61,49,74,0.5) divider
- Destructive color #ff6b6b, 14px/600 black section title

Add tokens: --color-settings-card-background, --color-settings-row-background,
--color-settings-divider, --color-chevron-icon, --color-destructive.

Wire the BlocConsumer listener equivalent: on currentUser transitioning to
null, dispatch ChatAuthStatusChanged + AuthReset and router.replace("/chat").

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

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

src/tokens/colors.css

@@ -83,4 +83,11 @@
 
   /* 底部弹层 drag handle */
   --color-auth-drag-handle: #3a3a3a;
+
+  /* Settings section (对齐 Flutter lib/ui/core/settings_section.dart) */
+  --color-settings-card-background: #ffffff;
+  --color-settings-row-background: #f5f5f5;
+  --color-settings-divider: #3d314a;
+  --color-chevron-icon: #000000;
+  --color-destructive: #ff6b6b;
 }