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/models/folder.rs

@@ -20,12 +20,13 @@ pub struct FolderDetail {
     pub parent_branch: Option<String>,
     pub default_agent_type: Option<AgentType>,
     pub last_opened_at: DateTime<Utc>,
-    pub opened_conversations: Vec<OpenedConversation>,
 }
 
 #[derive(Debug, Clone, Serialize, Deserialize)]
-pub struct OpenedConversation {
-    pub conversation_id: i32,
+pub struct OpenedTab {
+    pub id: i32,
+    pub folder_id: i32,
+    pub conversation_id: Option<i32>,
     pub agent_type: AgentType,
     pub position: i32,
     pub is_active: bool,

src-tauri/src/models/mod.rs

@@ -14,7 +14,7 @@ pub use conversation::{
     DbConversationDetail, DbConversationSummary, FolderInfo, ImportResult, SessionStats,
     SidebarData,
 };
-pub use folder::{FolderCommandInfo, FolderDetail, FolderHistoryEntry, OpenedConversation};
+pub use folder::{FolderCommandInfo, FolderDetail, FolderHistoryEntry, OpenedTab};
 pub use message::{
     AgentExecutionStats, AgentToolCall, ContentBlock, MessageRole, MessageTurn, TurnRole,
     TurnUsage, UnifiedMessage,