👷 bump version to 2.1.0

[guiyanakuang]

Summary

• Bump crosspaste-version.properties to 2.1.0.
• Add CHANGELOG.md entry for 2.1.0 covering the 78 commits since 2.0.0.2192.

Headline items:

• 🧊 Compose Multiplatform 1.11 upgrade (⬆️ bump Compose Multiplatform to 1.11.0 #4417 + jewel / richeditor / shimmer follow-ups)
• 🛡️ Windows Public network profile detection with one-click settings jump (Detect Windows Public network profile and offer one-click jump to settings #4402 🐛 fix COM ref leak and false-positive warning in Windows network detection #4404)
• 🔋 UserAttentionService gates background pollers on window visibility (✨ add UserAttentionService to gate background pollers on window visibility #4406 Harden launchWhileAttentive against unhandled exceptions in block #4414)
• 🖼️ Animated GIF + native video thumbnails in side panel, plus Skia close race fix (Preview video files in side panel with native thumbnails #4353 ✨ animate GIFs in image side preview via Skia codec #4410 ⚡ decode GIF frames off the UI thread with double buffering #4422 🐛 serialize Skia codec/bitmap close with in-flight readPixels #4438)
• 🧪 :e2e real-device test harness (✅ add :e2e real-device test harness #4318)

Test plan

• ./gradlew app:desktopTest (local) — BUILD SUCCESSFUL in 1m 8s
• CI app-build green
• After merge: git tag 2.1.0.$(git rev-list --count HEAD) on main and push, then watch build-release.yml