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/capabilities/default.json

@@ -2,7 +2,7 @@
   "$schema": "../gen/schemas/desktop-schema.json",
   "identifier": "default",
   "description": "Capability for the main window",
-  "windows": ["welcome", "folder-*", "commit-*", "merge-*", "stash-*", "push-*", "settings", "project-boot"],
+  "windows": ["main", "commit-*", "merge-*", "stash-*", "push-*", "settings", "project-boot"],
   "permissions": [
     "core:default",
     "core:window:default",

src-tauri/capabilities/desktop.json

@@ -6,8 +6,7 @@
     "linux"
   ],
   "windows": [
-    "welcome",
-    "folder-*",
+    "main",
     "commit-*",
     "merge-*",
     "stash-*",