Add connection status indicator

Show the current network connection status and RPC health in the UI.

**Acceptance Criteria:**
- Show green/red dot in Navbar indicating RPC connection status
- Display network name (Testnet/Mainnet) next to indicator
- Show warning banner when RPC is unreachable
- Auto-retry connection every 30 seconds

**Files:** new file `src/components/ConnectionStatus.tsx`, `src/components/Navbar.tsx`