feat: Add deep link and launcher shortcut support for conversations

[angrymuesli]

• Add custom URI scheme (nctalk://conversation/{token}) for opening conversations from external launchers like KISS
• Add HTTPS deep link support for /call/{token} URLs (fixes Open Talk links in App #847)
• Add dynamic shortcuts for favorite/recent conversations
• Add "Add to home screen" menu option in conversation long-press dialog
• New DeepLinkHandler utility for parsing deep link URIs
• New ShortcutManagerHelper utility for managing conversation shortcuts

🖼️ Screenshots

🏚️ Before	🏡 After
B	A

🚧 TODO

• ...

🏁 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?)

[sowjanyakch]

Thank you for the PR @angrymuesli
We will review your PR soon.

[github-actions]

Hello there,
Thank you so much for taking the time and effort to create a pull request to our Nextcloud project.

We hope that the review process is going smooth and is helpful for you. We want to ensure your pull request is reviewed to your satisfaction. If you have a moment, our community management team would very much appreciate your feedback on your experience with this PR review process.

Your feedback is valuable to us as we continuously strive to improve our community developer experience. Please take a moment to complete our short survey by clicking on the following link: https://cloud.nextcloud.com/apps/forms/s/i9Ago4EQRZ7TWxjfmeEpPkf6

Thank you for contributing to Nextcloud and we hope to hear from you soon!

(If you believe you should not receive this message, you can add yourself to the blocklist.)