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/src/db.rs

@@ -2,6 +2,16 @@ use chrono::{DateTime, Utc};
 use sqlx::FromRow;
 use uuid::Uuid;
 
+/// Extract @mentions from message content
+fn parse_mentions(content: &str) -> Vec<String> {
+    content
+        .split_whitespace()
+        .filter(|w| w.starts_with('@') && w.len() > 1)
+        .map(|w| w.trim_start_matches('@').trim_end_matches(|c: char| !c.is_alphanumeric() && c != '_' && c != '-').to_string())
+        .filter(|m| !m.is_empty())
+        .collect()
+}
+
 /// DB row types — these map directly to SQL tables.
 /// Separate from API types in colony-types.
 
@@ -87,6 +97,7 @@ impl MessageRow {
             } else {
                 self.content.clone()
             },
+            mentions: parse_mentions(&self.content),
             metadata: self
                 .metadata
                 .as_ref()