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/commands/folders.rs

@@ -567,6 +567,19 @@ pub async fn reorder_folders(
         .map_err(AppCommandError::from)
 }
 
+#[cfg(feature = "tauri-runtime")]
+#[cfg_attr(feature = "tauri-runtime", tauri::command)]
+pub async fn update_folder_color(
+    db: tauri::State<'_, AppDatabase>,
+    folder_id: i32,
+    color: String,
+) -> Result<crate::models::FolderDetail, AppCommandError> {
+    folder_service::update_folder_color(&db.conn, folder_id, &color)
+        .await
+        .map_err(AppCommandError::from)?
+        .ok_or_else(|| AppCommandError::not_found("Folder not found"))
+}
+
 #[cfg_attr(feature = "tauri-runtime", tauri::command)]
 pub async fn create_folder_directory(path: String) -> Result<(), AppCommandError> {
     std::fs::create_dir_all(&path).map_err(AppCommandError::io)