feat(conversations): offer the webinar, presentation and classified conversation types

[AndyScherzinger]

The three remaining conversation types the server offers are now available when creating a
conversation, together with the settings each of them applies:

• Webinar — public, with the lobby enabled, restricted participant permissions and recording consent required.
• Presentation — restricted permissions for internal presentations, recording consent required.
• Classified — locked down for confidential topics.

Classified conversations get the client side handling they need: their visibility can not be changed
and only users, groups and teams can be invited, so participants the server would reject are dropped
when the type is selected. The server coerces all of this at creation time as well, so this is the
honest reflection of what will happen rather than the only barrier.

A summary below the cards lists the settings the selected type applies, leaving out the ones an
administrator pinned for every conversation. Creating a public conversation now offers to share its
link and to copy its password. And a conversation that was created without all of its participants,
or not at all, no longer fails silently — creation errors were previously invisible, which left an
enabled Create button and invited a duplicate conversation.

This PR is stacked on #6650 and targets its branch, not master. GitHub retargets it as the
earlier ones merge. It is the last of the three; with it the app offers every conversation type the
web does.

🖼️ Screenshots

🏚️ Before	🏡 After
B	A

🚧 TODO

• screenshots of the new types, the settings summary, a classified conversation with its locked visibility, and the share dialog — light and dark, and with a non-default server primary colour

🏁 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?)

Capabilities checked: conversation-presets gates the type selection, classified-conversations
and announcement-preset gate the respective types, and the presets endpoint decides which types an
installation offers at all.

🤖 AI (if applicable)

• The content of this PR was partly or fully generated using AI

🤖 Generated with Claude Code

https://claude.ai/code/session_014YnDswTbCmRrVETnwE4twB

[AndyScherzinger]

/backport to stable-25.0.x

[github-actions]

APK file: https://github.com/nextcloud/talk-android/actions/runs/34147812701/artifacts/10028583648
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)