refactor(panels): read panel state from the panel chrome store

PanelChrome, its collapse-preference restore and the phone shell read a
panel's disclosure state back from the gev-panel-chrome store
(_shownPanel), falling back to the markup's classes before the panel is
first presented, instead of reading the classes v-panel-state draws. The
share link and the phone shell share one rule for a panel the person keeps
closed (prefersCollapsed). The rails, the accordion and the hover
disclosure still read the classes, which the directive draws in the same
task as their layout.

docs/VUE-MIGRATION.md

@@ -124,7 +124,13 @@ a feature supplying only its accent.
    `accordion-collapsed`): PanelChrome, the rails and the accordion present
    it rather than writing classes. The directive applies it in a synchronous
    watcher, not on Vue's next render, because the rails measure a panel in
-   the same task that opens or closes it. Done.
+   the same task that opens or closes it. PanelChrome and its preference
+   restore, and the phone shell, read a panel's state back from the store
+   (`_shownPanel`; the markup's classes before its first presentation), so
+   once a panel is presented a class written outside the store no longer
+   counts. The rails, the accordion and the hover disclosure
+   (`panelDisclosure.js`), which act within the same task as a layout, read
+   the classes the directive has just drawn. Done.
 3. **Panels.** Each template and its wiring become a component, keeping ids,
    classes and CSS; text a template renders with `t()` drops its `data-i18n`
    marker. The page places each view with a