✨ refactor(web/default): adopt drill-in sidebar pattern for System Settings

Replace the ad-hoc "workspace" abstraction with a focused, URL-driven
"sidebar view" registry that implements the modern Vercel / Cloudflare
drill-in pattern: clicking a top-level entry (e.g. System Settings)
swaps the sidebar to a contextual workspace, with a `← Back to
Dashboard` affordance, instead of stacking sub-navigation in the root.

Architecture
------------
- types.ts
    + SidebarView           — declarative nested view config
                              (id, pathPattern, parent, getNavGroups)
    + SidebarViewParent     — back-navigation descriptor
    + ResolvedSidebarView   — { key, view, navGroups } returned by hook
    + SidebarData           — slimmed to { navGroups } only
    - Workspace             — removed (logo/plan never rendered)

- lib/sidebar-view-registry.ts (new, replaces workspace-registry.ts)
    + SIDEBAR_VIEWS array — single source of truth for nested views
    + resolveSidebarView(pathname)
    + getNavGroupsForPath(pathname, t) — back-compat helper for the
      command palette

- config/system-settings.config.ts
    Refactored to export a single SYSTEM_SETTINGS_VIEW (SidebarView)
    with parent `/dashboard/overview` + label `Back to Dashboard`.

- components/sidebar-view-header.tsx (new)
    Renders only the back affordance (chevron + label). Uses the
    default SidebarMenuButton size so its typography matches the
    nav items below; collapses gracefully into icon mode via the
    existing tooltip behavior. The redundant "title + icon" row was
    removed — workspace context is already carried by the nav groups.

- hooks/use-sidebar-view.ts (new)
    Encapsulates view resolution and root-nav filtering:
      · matched view  → returns its nav groups verbatim (route-level
                        beforeLoad guards already enforce access);
      · no match      → returns root nav groups, narrowed by user
                        role (admin gate) and useSidebarConfig
                        (admin × user sidebar_modules overlay).

- components/app-sidebar.tsx
    Now a thin presentation layer: reads { key, view, navGroups }
    from useSidebarView() and orchestrates the view transition via
    AnimatePresence + MOTION_VARIANTS.sidebarSlide (respects
    prefers-reduced-motion). No logic, no role checks, no path
    matching — those live in the hook.

- components/command-menu.tsx
    Switched to the new getNavGroupsForPath() API; behavior preserved.

Cleanup
-------
- Deleted layout/context/workspace-context.tsx (zero consumers).
- Deleted layout/lib/workspace-registry.ts and its
  workspace-registry.example.ts companion (over-abstracted: name/id
  metadata, isInWorkspace / getAllWorkspaces / WORKSPACE_IDS were
  registered but never read).
- Removed `workspaces` field from useSidebarData (never consumed
  after the top-switcher was dropped).
- Dropped WorkspaceProvider from authenticated-layout.tsx.
- Trimmed dead `Manage and configure` translation key from all six
  locale files and from static-keys.ts.

i18n
----
Added the `Back to Dashboard` key to en, zh, fr, ja, ru, vi, and
registered it in static-keys.ts under "Sidebar views".

Verification
------------
- bun run typecheck: passes
- Lint: no new warnings/errors on the touched files
- Adding a new drill-in workspace now only requires registering a
  SidebarView in SIDEBAR_VIEWS — no changes to AppSidebar required.

web/default/src/i18n/locales/zh.json

@@ -469,6 +469,7 @@
     "Azure": "Azure",
     "AZURE_OPENAI_ENDPOINT *": "AZURE_OPENAI_ENDPOINT *",
     "Back": "返回",
+    "Back to Dashboard": "返回控制台",
     "Back to Home": "返回主页",
     "Back to login": "返回登录",
     "Back to Models": "返回模型",
@@ -2241,7 +2242,6 @@
     "Make it easier for teammates to pick the right group.": "让队友更容易选择正确的分组。",
     "Manage": "管理",
     "Manage account bindings for this user": "管理此用户的账户绑定",
-    "Manage and configure": "管理和配置",
     "Manage API channels and provider configurations": "管理 API 渠道和提供商配置",
     "Manage Bindings": "管理绑定",
     "Manage catalog visibility and pricing.": "管理目录可见性和定价。",