add @mentions — parsed server-side, rendered as highlighted spans

- Backend: parse_mentions() extracts @username from content
- Message API response includes mentions: string[] field
- Frontend: renderContent() highlights @mentions in hot orange
- Works with alphanumeric + hyphens + underscores

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

crates/colony-types/src/lib.rs

@@ -44,6 +44,7 @@ pub struct Message {
     pub content: String,
     #[ts(optional)]
     pub metadata: Option<serde_json::Value>,
+    pub mentions: Vec<String>,
     #[ts(optional)]
     pub reply_to: Option<Uuid>,
     pub created_at: DateTime<Utc>,