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/i18n/messages/en.json

@@ -782,7 +782,8 @@
         "folderRemoved": "Removed folder {name}",
         "openFolderFailed": "Failed to open folder",
         "removeFolderFailed": "Failed to remove folder: {message}",
-        "reorderFoldersFailed": "Failed to reorder folders: {message}"
+        "reorderFoldersFailed": "Failed to reorder folders: {message}",
+        "changeFolderColorFailed": "Failed to change folder color: {message}"
       },
       "statsLabel": "{folders} folders · {convos} conversations",
       "reorderHandle": "Drag to reorder",
@@ -802,6 +803,7 @@
       "removeFolderConfirmDescription": "Remove \"{name}\" from the workspace? Its tabs and terminals will close.",
       "folderHeaderMenu": {
         "manageConversations": "Manage conversations…",
+        "changeColor": "Change color",
         "removeFromWorkspace": "Remove from workspace"
       },
       "manageConversations": {