Backend:
- Add MENTION_TAG_RE matching new `<mention type="..." id="...">label</mention>` format
- Extend extract_mentions() and resolve_mentions() to parse new format (legacy backward-compatible)
Frontend:
- New src/lib/mention-ast.ts: AST types (TextNode, MentionNode, AiActionNode),
parse() and serialize() functions for AST↔HTML conversion
- MentionPopover: load @repository: from project repos, @ai: from room_ai configs
(not room members); output new HTML format with ID instead of label
- MessageMentions: use AST parse() for rendering (falls back to legacy parser)
- ChatInputArea: insertMention now produces `<mention type="user" id="...">label</mention>`
- RoomParticipantsPanel: onMention passes member UUID to insertMention
- RoomContext: add projectRepos and roomAiConfigs for mention data sources
