refactor(workspace): migrate from per-folder windows to single-window workspace

Replace the legacy folder + welcome routes with a unified /workspace route
that hosts all folders, conversations, tabs, and terminals in one window.

- Persist opened tabs to the database (opened_tabs entity + migration)
  so tab layout survives restarts and deep-link bootstrap restores state
- Replace FolderContext shim with AppWorkspaceProvider, ActiveFolderProvider,
  and TabProvider; expose both opened (folders) and full DB (allFolders)
  listings via list_all_folder_details
- Return conversations across all non-deleted folders from list_all when
  no folder filter is given, so the sidebar can show every folder's history
- Add ConversationContextBar above the chat input with folder picker
  (auto-opens unopened folders on select), branch picker, and commit /
  push / merge / stash entries to restore BranchDropdown functionality
- Rework sidebar with stats header, search, flat / folder-grouped view
  modes (localStorage-persisted), reveal-in-sidebar event subscriber,
  and per-folder context menu (focus, close tabs, remove from workspace);
  indent conversations under folder headers in grouped mode
- Gate terminal creation on active folder and show folder context
- Remove deprecated BranchDropdown, FolderNameDropdown, welcome route,
  and per-folder window commands
- Localize all new strings across 10 locales

src-tauri/src/db/migration/mod.rs

@@ -9,6 +9,7 @@ mod m20260330_000001_chat_channel;
 mod m20260401_000001_chat_channel_sender_context;
 mod m20260404_000001_model_provider;
 mod m20260406_000001_agent_setting_model_provider;
+mod m20260420_000001_opened_tabs;
 pub struct Migrator;
 
 #[async_trait::async_trait]
@@ -24,6 +25,7 @@ impl MigratorTrait for Migrator {
             Box::new(m20260401_000001_chat_channel_sender_context::Migration),
             Box::new(m20260404_000001_model_provider::Migration),
             Box::new(m20260406_000001_agent_setting_model_provider::Migration),
+            Box::new(m20260420_000001_opened_tabs::Migration),
         ]
     }
 }