feat: add conversation categories

[Antreesy]

☑️ Resolves

• Ref Sorting and grouping of Talk Chats #12025
• Custom sections: Users can create named sections to organize conversations in the sidebar. Sections can be created, renamed, deleted, and reordered via drag-and-drop in the "Manage sections" dialog.
• Section behavior: Favorites are always pinned at the top, unsectioned conversations appear in an "Other" group at the bottom. Sections are collapsible with unread count badges.

AI (if applicable)

• The content of this PR was partly or fully generated using AI
• Extracted from community PR feat(sidebar): Add conversation sections and sort options #17368

🖌️ UI Checklist

🖼️ Screenshots / Screencasts

🏚️ Before	🏡 After
Screenshot before

🚧 Tasks

• Create, rename, and delete sections
• Assign conversations to sections via context menu
• Reorder sections via drag-and-drop / arrows
• Collapse/expand sections, verify unread count badge shows on collapsed sections
• Verify favorites always appear at top, unsectioned in "Other" at bottom
• Allow to move 'favorites' (?) and 'others' categories, treat as such
• Verify favorite / archived conversations can be assigned to sections
• Implement collapsed with server or BrowseStorage?
• Wording (tags/labels/categories)
• Move virtual headings from LeftSidebar to ConversationVirtualList
• Add typings to new service / store

🏁 Checklist

• 🌏 Tested with different browsers / clients:
  • Chromium (Chrome / Edge / Opera / Brave)
  • Firefox
  • Safari
  • Talk Desktop
  • Integrations with Files sidebar and other apps
  • Not risky to browser differences / client
• 🖌️ Design was reviewed, approved or inspired by the design team
• ⛑️ Tests are included or not possible
• 📗 User documentation in https://github.com/nextcloud/documentation/tree/master/user_manual/talk has been updated or is not required

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🛠️ API Checklist

• New talk_conversation_sections table with id, user_id, name, sort_order, collapsed columns
• New ConversationSectionController with full CRUD + reorder API (/api/v4/sections)
• New sectionId field on Attendee model to assign conversations to sections
• New capability: conversation-sections

🚧 Tasks

• decide on whether to store collapsed or not
• support favorites/others as hardcoded category ids for moving order

🏁 Checklist

• ⛑️ Tests (unit and/or integration) are included or not possible
• 📘 API documentation in docs/ has been updated or is not required
• 🔖 Capability is added or not needed