Add useSessionSync hook to detect and synchronize wallet session changes
across browser tabs and wallet provider account switches. Ensures wallet
logout or account switch in one tab is reflected in all other tabs.

New components:
- useSessionSync hook: Listens for storage events and wallet provider
  changes to detect session mutations in other tabs

Detects:
- Storage events (localStorage changes from other tabs)
- Wallet provider account changes (Leather, Xverse)
- Session logout/login from other tabs
- Account switching via provider UI

Implementation:
- Registers storage event listener on mount for blockstack-session key
- Registers provider accountsChanged event listeners for both Leather
  and Xverse extensions
- Invokes callback with updated session state when changes detected
- Cleans up all listeners on unmount to prevent memory leaks

Updated App.jsx:
- Added useSessionSync to recompute session state on cross-tab changes
- Session restoration now happens both on mount and on external changes
- Auth loading state properly reset when session changes

Testing:
- 12 comprehensive unit tests covering all scenarios
  - Storage events (logout, login, account switch)
  - Provider account change events
  - Listener lifecycle (mount/unmount)
  - Multiple rapid changes
  - Edge cases (null key, other keys)
- All 1724 frontend tests passing
- All 91 contract tests passing

Benefits:
- Users see immediate logout if they disconnect in another tab
- Account switching is instantly reflected
- No stale wallet addresses visible
- Works across multiple browser windows/tabs
- Gracefully handles providers without event support

Fixes #290