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/entities/opened_tab.rs

@@ -0,0 +1,47 @@
+use sea_orm::entity::prelude::*;
+
+#[derive(Clone, Debug, PartialEq, DeriveEntityModel)]
+#[sea_orm(table_name = "opened_tab")]
+pub struct Model {
+    #[sea_orm(primary_key)]
+    pub id: i32,
+    pub folder_id: i32,
+    pub conversation_id: Option<i32>,
+    pub agent_type: String,
+    pub position: i32,
+    pub is_active: bool,
+    pub is_pinned: bool,
+    pub created_at: DateTimeUtc,
+    pub updated_at: DateTimeUtc,
+}
+
+#[derive(Copy, Clone, Debug, EnumIter, DeriveRelation)]
+pub enum Relation {
+    #[sea_orm(
+        belongs_to = "super::folder::Entity",
+        from = "Column::FolderId",
+        to = "super::folder::Column::Id"
+    )]
+    Folder,
+
+    #[sea_orm(
+        belongs_to = "super::conversation::Entity",
+        from = "Column::ConversationId",
+        to = "super::conversation::Column::Id"
+    )]
+    Conversation,
+}
+
+impl Related<super::folder::Entity> for Entity {
+    fn to() -> RelationDef {
+        Relation::Folder.def()
+    }
+}
+
+impl Related<super::conversation::Entity> for Entity {
+    fn to() -> RelationDef {
+        Relation::Conversation.def()
+    }
+}
+
+impl ActiveModelBehavior for ActiveModel {}