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/m20260420_000001_opened_tabs.rs

@@ -0,0 +1,132 @@
+use sea_orm_migration::prelude::*;
+
+#[derive(DeriveMigrationName)]
+pub struct Migration;
+
+#[async_trait::async_trait]
+impl MigrationTrait for Migration {
+    async fn up(&self, manager: &SchemaManager) -> Result<(), DbErr> {
+        manager
+            .drop_table(
+                Table::drop()
+                    .table(FolderOpenedConversation::Table)
+                    .if_exists()
+                    .to_owned(),
+            )
+            .await?;
+
+        manager
+            .create_table(
+                Table::create()
+                    .table(OpenedTab::Table)
+                    .if_not_exists()
+                    .col(
+                        ColumnDef::new(OpenedTab::Id)
+                            .integer()
+                            .not_null()
+                            .auto_increment()
+                            .primary_key(),
+                    )
+                    .col(ColumnDef::new(OpenedTab::FolderId).integer().not_null())
+                    .col(ColumnDef::new(OpenedTab::ConversationId).integer().null())
+                    .col(ColumnDef::new(OpenedTab::AgentType).string().not_null())
+                    .col(ColumnDef::new(OpenedTab::Position).integer().not_null())
+                    .col(
+                        ColumnDef::new(OpenedTab::IsActive)
+                            .boolean()
+                            .not_null()
+                            .default(false),
+                    )
+                    .col(
+                        ColumnDef::new(OpenedTab::IsPinned)
+                            .boolean()
+                            .not_null()
+                            .default(false),
+                    )
+                    .col(
+                        ColumnDef::new(OpenedTab::CreatedAt)
+                            .timestamp_with_time_zone()
+                            .not_null(),
+                    )
+                    .col(
+                        ColumnDef::new(OpenedTab::UpdatedAt)
+                            .timestamp_with_time_zone()
+                            .not_null(),
+                    )
+                    .foreign_key(
+                        ForeignKey::create()
+                            .from(OpenedTab::Table, OpenedTab::FolderId)
+                            .to(Folder::Table, Folder::Id)
+                            .on_delete(ForeignKeyAction::Cascade),
+                    )
+                    .foreign_key(
+                        ForeignKey::create()
+                            .from(OpenedTab::Table, OpenedTab::ConversationId)
+                            .to(Conversation::Table, Conversation::Id)
+                            .on_delete(ForeignKeyAction::Cascade),
+                    )
+                    .to_owned(),
+            )
+            .await?;
+
+        manager
+            .create_index(
+                Index::create()
+                    .name("idx_opened_tabs_folder_id")
+                    .table(OpenedTab::Table)
+                    .col(OpenedTab::FolderId)
+                    .to_owned(),
+            )
+            .await?;
+
+        manager
+            .create_index(
+                Index::create()
+                    .name("idx_opened_tabs_position")
+                    .table(OpenedTab::Table)
+                    .col(OpenedTab::Position)
+                    .to_owned(),
+            )
+            .await?;
+
+        Ok(())
+    }
+
+    async fn down(&self, manager: &SchemaManager) -> Result<(), DbErr> {
+        manager
+            .drop_table(Table::drop().table(OpenedTab::Table).if_exists().to_owned())
+            .await?;
+        Ok(())
+    }
+}
+
+#[derive(DeriveIden)]
+enum OpenedTab {
+    Table,
+    Id,
+    FolderId,
+    ConversationId,
+    AgentType,
+    Position,
+    IsActive,
+    IsPinned,
+    CreatedAt,
+    UpdatedAt,
+}
+
+#[derive(DeriveIden)]
+enum FolderOpenedConversation {
+    Table,
+}
+
+#[derive(DeriveIden)]
+enum Folder {
+    Table,
+    Id,
+}
+
+#[derive(DeriveIden)]
+enum Conversation {
+    Table,
+    Id,
+}

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),
         ]
     }
 }