fix(tonconnect): use 'browser' platform for extension

[mois-ilya]

Summary

• Add centralized getTonConnectPlatform(targetEnv) function for TonConnect DeviceInfo platform
• Extension returns 'browser', other environments use browser detection
• Make getBrowserPlatform private (internal use only)

Changes

• core: Add getTonConnectPlatform() in connectService.ts
• uikit: Use getTonConnectPlatform(sdk.targetEnv) in TonConnectNotification
• extension: Use getTonConnectPlatform('extension') in both TonConnect files
• mobile: Use getTonConnectPlatform('mobile') for consistency

Why

Platform determination was duplicated and inconsistent. Now there's a single source of truth.

Fixes PRO-411

[github-actions]

Successful WEB deployment 🚀🚀🚀

Well done!
Link to test environment:
https://681f4f2a.tonkeeper-web.pages.dev

[BAHRALMN16]

اهلا بك يااخي وشكرا لك ولاكن انا لا لا أفهم اينه تكون الملفات 📂 التي تتكلم عنها ممكن توضيح أكثر وشكرا علا جهجهودكم

[Anonym-tsk]

lgtm