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/db/migration/mod.rs

@@ -12,6 +12,7 @@ mod m20260406_000001_agent_setting_model_provider;
 mod m20260420_000001_opened_tabs;
 mod m20260422_000001_folder_sort_order;
 mod m20260423_000001_drop_folder_parent_branch;
+mod m20260424_000001_folder_color;
 pub struct Migrator;
 
 #[async_trait::async_trait]
@@ -30,6 +31,7 @@ impl MigratorTrait for Migrator {
             Box::new(m20260420_000001_opened_tabs::Migration),
             Box::new(m20260422_000001_folder_sort_order::Migration),
             Box::new(m20260423_000001_drop_folder_parent_branch::Migration),
+            Box::new(m20260424_000001_folder_color::Migration),
         ]
     }
 }