feat(emulator): disconnection alerts, backoff auto-reconnect, loading indicators

[Copilot]

The emulator view had no feedback when a streaming connection dropped unexpectedly, no loading visuals during metadata fetch or WebRTC connect, and no recovery mechanism for transient disconnects.

Changes

emulator-error.tsx — new DisconnectionBanner + DisconnectionInfo type

• DisconnectionInfo: shared type carrying { countdown, attempt, maxAttempts, failed }
• DisconnectionBanner: accessible role="alert" banner with live countdown, attempt/max display, "Reconnect Now" (skip countdown) and "Dismiss" (cancel reconnect) actions

emulator.css — .emu-spinner

• Lightweight CSS-only spinner (@keyframes emu-spin) usable inline in buttons and placeholders

webrtc-emulator-view.tsx

• Connect button renders spinner + "Connecting…" while negotiating; canvas placeholder also shows spinner instead of static text
• handleStateChange distinguishes user-initiated vs unexpected disconnect via userDisconnectRef; unexpected "disconnected" triggers handleUnexpectedDisconnect
• Exponential backoff reconnect (1 → 2 → 4 → 8 → 16s, capped at 30s, max 5 attempts); all timer state held in refs to avoid stale closures; clearReconnectTimers() helper used consistently
• DisconnectionBanner displayed with live countdown; unmount effect cancels pending timers

adb-emulator-view.tsx

• Connect button shows spinner + "Loading metadata…" while isLoadingMeta
• "No devices" banner is context-aware: initial guidance / post-connect no-devices message / loading state
• Same backoff reconnect strategy wired to the WebSocket onclose handler; disconnect() sets userDisconnectRef.current = true to suppress reconnect on intentional close
• DisconnectionBanner rendered above the main emulator area

// Backoff: 1s → 2s → 4s → 8s → 16s, ceiling 30s, 5 attempts max
const delayMs = Math.min(INITIAL_BACKOFF_MS * Math.pow(2, attempt - 1), MAX_BACKOFF_MS);