feat(tabs): pin tabs to a Focus section on the vertical rail

A session or web-tab ref in the owner tab layout may now carry
`focus: true`. "Add to Focus" in a row's menu (right-click on a web tab)
sets it; the vertical rail then draws a shortcut to that tab in a pinned,
collapsible "Focus" section at its top, while the row itself stays where
it is. A click on a shortcut selects its tab, a right-click or the menu
key offers "Remove from Focus".

Model: `TabRef.focus?: true`, parsed strictly (only `true` or absent) and
carried through normalization, every pure edit and the legacy-order
projection; `setTabFocus()` is the pure helper. TabLayoutService needed
no change and no route was added: the flag rides the existing
`PUT /api/tab-layout`. Browser: `copyRef`/`normalizeLayout` keep `focus`
the way they keep `placement` (the browser writes whole layouts, so a
dropped field would be erased on every save), `setFocus` is a named
operation through `createEditCoordinator` and replays on a 409, and
`focusRefs()` (also on `project()`) lists what the rail can draw.

The section is its own element in #tabRail above #sessionTabs, so it
works on the flat rail (no named groups) as well as the grouped one, and
shortcuts are never rows: no Alt+N badge, no tree item, no lineage
anchor, no drag target. It is a list with one tab stop and
Up/Down/Home/End between shortcuts. Collapse is per-device localStorage
under its own key. A shortcut whose session closed or whose web tab is
not open drops out; the server already prunes deleted refs. zh-CN
strings added.

config/test-suites.ts

@@ -23,6 +23,7 @@ export const BROWSER_TEST_GLOBS = [
   'test/tab-activation.browser.test.ts',
   'test/tab-layout-editing.browser.test.ts',
   'test/tab-rail-search.browser.test.ts',
+  'test/tab-focus.browser.test.ts',
   'test/session-sidebar-ux.browser.test.ts',
   'test/session-options-responsive.browser.test.ts',
   'test/inline-rename.test.ts',