feat(tabs): grouped vertical rail from owner tab layouts

The vertical tab rail now reads the owner's tab layout (GET /api/tab-layout)
and draws its groups as collapsible sections. This is the first frontend
consumer of the tab-layout backend and it is read-only: nothing in the
browser writes the layout yet.

- tab-layout-browser.js (new, pure, loaded before app.js): projects the
  layout onto the live sessions and open web tabs, renders the grouped
  markup, stores collapse per device, and sequences loads newest-wins with
  a bounded retry on failure.
- app.js: loads the layout on init and on tab:layoutChanged, renders the
  grouped rail from the same per-row markup the flat rail uses, falls
  through to a full render whenever the grouping structure changes, and
  withholds drag-reorder in the grouped rail.
- Grouping is opt-in by construction. With no layout, a failed read, a
  layout without groups, or a horizontal strip, the rail renders exactly
  as before (byte-identical markup).
- Grouping is a render layer only: sessionOrder, Alt+N, Ctrl+Tab and the
  palette keep reading the server-projected order, and row badges keep
  their Alt+N slot.
- A collapsed group still shows the active row; lineage arcs to a hidden
  session anchor to its group header.
- webview-tabs.js: renderWebviewTab() extracted so a single web tab can be
  placed into its group with unchanged markup.

src/web/public/i18n.js

@@ -67,6 +67,7 @@
     'Open away digest': '打开离开期间摘要',
     'Session Manager': '会话管理器',
     'Session actions': '会话操作',
+    Ungrouped: '未分组',
     'Open session manager': '打开会话管理器',
     Attachments: '附件',
     'Open attachment history': '打开附件历史',