feat(ui): render the Context mode switch from a store

ContextControls presents the selected mode, the transition busy state and the nearby sites search through a publish option, and ContextView renders them from the gev-context store. ./ui/context stays free of Vue. The Radio QA reads the search button's aria-busy, the state it actually carries.

src/ui/shellElements.js

@@ -85,9 +85,6 @@ export function readShellElements(document = globalThis.document) {
     _globalContextMissionsBtn: document.getElementById(
       'global-context-missions-btn',
     ),
-    _contextModeStandby: document.getElementById('context-mode-standby'),
-    _contextFlightsView: document.getElementById('context-flights-view'),
-    _contextMissionsView: document.getElementById('context-missions-view'),
     _installationsSearchBtn: document.getElementById(
       'installations-search-btn',
     ),