feat: QR code scanning for gateway onboarding

iOS:
- QR scanner view using DataScannerViewController
- Photo library QR detection via CIDetector for saved QR images
- Deep link parser for openclaw://gateway URLs and base64url setup codes
- Onboarding wizard: full-screen welcome with "Scan QR Code" button,
  auto-connect on scan, back navigation, step indicators for manual flow

Backend:
- Add /pair qr action to device-pair extension for QR code generation
- TUI/WebUI differentiation: ASCII QR for TUI, markdown image for WebUI
- Telegram: send QR as media attachment via sendMessageTelegram
- Add data URI support to loadWebMedia for generic base64 media handling
- Export renderQrPngBase64 from plugin SDK for extension use

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
(cherry picked from commit d79ed65be0)

src/plugin-sdk/index.ts

@@ -451,3 +451,6 @@ export type { ProcessedLineMessage } from "../line/markdown-to-line.js";
 
 // Media utilities
 export { loadWebMedia, type WebMediaResult } from "../web/media.js";
+
+// QR code utilities
+export { renderQrPngBase64 } from "../web/qr-image.js";