fix(voice): improve slide-to-cancel UX for voice message recording

[mahibi]

• Fix swipe speed: text now follows finger 1:1 instead of using a fixed increment per touch event
• Start tracking swipe from the initial touch-down, eliminating the dead zone while the finger is still over the record button
• Double the cancel threshold (150 → 300px) so accidental cancellations are less likely
• Center the slide-to-cancel text with a 30dp rightward offset
• Add a fade-out gradient to the recording timer so the sliding text disappears smoothly behind it instead of overlapping visibly; separate light/dark drawables avoid the grey transparent artifact

AI-assistant: Claude Code v2.1.142 (Claude Sonnet 4.6)

🏁 Checklist

• ⛑️ Tests (unit and/or integration) are included or not needed
• 🔖 Capability is checked or not needed
• 🔙 Backport requests are created or not needed: /backport to stable-xx.x
• 📅 Milestone is set
• 🌸 PR title is meaningful (if it should be in the changelog: is it meaningful to users?)

[mahibi]

/backport to stable-24.0

[github-actions]

APK file: https://github.com/nextcloud/talk-android/actions/runs/26824547396/artifacts/7360631159
To test this change/fix you can simply download above APK file and install and test it in parallel to your existing Nextcloud app.
(please click on link to get QR code displayed)