fix(mobile): navigation back behavior, scroll button position

- New Chat → Chat uses replaceState so back skips new chat screen
- Active tab persisted to sessionStorage, restored on back navigation
- Scroll-to-bottom button positioned relative to footer (not hardcoded)
- overscroll-behavior: none to prevent iOS rubber-band

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/lib/storage-keys.ts

@@ -5,6 +5,7 @@ export const STORAGE = {
   PROJECT_DIR: 'clawtap:projectDir',
   DRAFT: 'clawtap:draft',
   INSTALL_DISMISSED: 'clawtap:install-dismissed',
+  SESSIONS_TAB: 'clawtap:sessionsTab',
   adapterPrefs: (id: string) => `clawtap:adapterPrefs:${id}` as const,
 } as const;