feat(sidebar): add per-folder color swatch with picker and neutral conversation rail

- Add `color` column to folder table with migration backfill and hash-based assignment on folder creation
- Expose `update_folder_color` via Tauri command and `/update_folder_color` HTTP route
- Render a color swatch before each folder name in the sidebar header; offer a 10-color palette (9 hues plus a theme-aware foreground sentinel) through the folder context menu
- Show the folder header "new conversation" button only on hover
- Drop the expanded-state tint on folder name and count badge; use a fixed neutral rail color for conversation items

src-tauri/src/models/folder.rs

@@ -20,6 +20,7 @@ pub struct FolderDetail {
     pub default_agent_type: Option<AgentType>,
     pub last_opened_at: DateTime<Utc>,
     pub sort_order: i32,
+    pub color: String,
 }
 
 #[derive(Debug, Clone, Serialize, Deserialize)]