feat(calendars): account header overflow menu + restyle

Replace the per-account toggle switch and nested manage row with a single
trailing overflow (⋮) menu holding both account-level actions:
- "Enable all" / "Disable all" — toggle every calendar in the group
- "Manage in app" (synced) / "Add calendar" (local)

The dropdown is styled to fit: rounded corners, a distinct floating
surface (surfaceContainerLowest + lifted shadow) so it stands clear of
the cards, and a divider separating the two actions. Shortened the manage
label to "Manage in app" and dropped the now-unused account a11y strings.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

app/src/main/res/values/strings.xml

@@ -344,11 +344,12 @@
     <string name="calendars_add">Add calendar</string>
     <string name="calendars_disable_hint">Turn a calendar off to remove it from the app — its events, filters and pickers. Nothing is deleted, and you can turn it back on here anytime.</string>
     <string name="calendars_show_in_app_a11y">Show \"%1$s\" in the app</string>
-    <string name="calendars_account_toggle_all_a11y">Show all calendars from %1$s in the app</string>
     <string name="calendars_synced_header">Synced calendars</string>
     <string name="calendars_synced_hint">These come from accounts on your device. Create and edit them in their own app.</string>
-    <string name="calendars_manage_in_app">Manage</string>
-    <string name="calendars_manage_account_a11y">Manage %1$s in its app</string>
+    <string name="calendars_manage_in_app">Manage in app</string>
+    <string name="calendars_account_menu_a11y">More options for %1$s</string>
+    <string name="calendars_enable_all">Enable all</string>
+    <string name="calendars_disable_all">Disable all</string>
     <string name="calendars_add_account">Add account</string>
     <string name="calendars_new_title">New calendar</string>
     <string name="calendars_edit_title">Edit calendar</string>