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/components/layout/status-bar-stats.tsx

@@ -3,7 +3,7 @@
 import { useMemo } from "react"
 import { BarChart3 } from "lucide-react"
 import { useTranslations } from "next-intl"
-import { useFolderContext } from "@/contexts/folder-context"
+import { useAppWorkspace } from "@/contexts/app-workspace-context"
 import { AGENT_LABELS } from "@/lib/types"
 import { AgentIcon } from "@/components/agent-icon"
 import {
@@ -14,7 +14,7 @@ import {
 
 export function StatusBarStats() {
   const t = useTranslations("Folder.statusBar.stats")
-  const { stats } = useFolderContext()
+  const { stats } = useAppWorkspace()
 
   const activeAgents = useMemo(
     () => stats?.by_agent.filter((a) => a.conversation_count > 0) ?? [],