Fix the blank VPN tab, and stop one page taking the console with it

On a machine with wireguard-tools installed but no tunnel up — core, which is
the machine the console opens on — the tunnel list was a nil slice, which is
null in JSON, and the page took its length. That throws, React unmounts the
whole tree, and what is left is a blank window with nothing to go on.

Two fixes, because either alone leaves the failure possible. The parse returns
an empty list rather than nil, with a test that pins the marshalled shape; and
the page treats a missing list as an empty one, so a node still running an older
build cannot blank the console either.

And the reason a small mistake had such a large blast radius: nothing caught it.
Each page now renders inside a boundary, so a fault costs that page and says
what it was, while the rail, the switcher and every other tab carry on. The
terminals are deliberately left outside it — they hold live WebSockets, and
remounting them to recover from somebody else's error would drop the sessions.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

web/src/Shell.tsx

@@ -27,7 +27,7 @@ import { MachineStatus, setMachine } from './api'
 import { LIST_FLOOR, useRefreshMs } from './prefs'
 import Power from './Power'
 import { applyAccent } from './accents'
-import { Btn, Toasts, askText, askYes, hasUnsaved, okToLeave, toast } from './ui'
+import { Btn, PageBoundary, Toasts, askText, askYes, hasUnsaved, okToLeave, toast } from './ui'
 import { fmtLeft, useTimeLeft } from './guest'
 
 // Inline outline icon so it matches the monochrome glyphs in the rail (emoji would be full colour).
@@ -320,6 +320,7 @@ export default function Shell({ me: hubMe, onLoggedOut }: { me: Me; onLoggedOut:
         <div className="backdrop" onClick={() => setDrawer(false)} />
 
         <main className="main" key={machine ?? 'hub'}>
+          <PageBoundary page={GROUPS.find((g) => g.id === view)?.label ?? view}>
           {view === 'settings' && <Settings me={hubMe} />}
           {view === 'users' && <Users me={hubMe} />}
           {view === 'kvm' && <KVM key={machine ?? 'hub'} machine={machine} title={title} />}
@@ -338,6 +339,7 @@ export default function Shell({ me: hubMe, onLoggedOut }: { me: Me; onLoggedOut:
           {view === 'vpn' && <VPN />}
           {view === 'zfs' && <ZFS />}
           {view === 'files' && <Files onOpenInTerminal={runInTerminal} />}
+          </PageBoundary>
           {termView && <TermTabs terms={ordered} active={active} creating={creating}
             onSelect={setActive} onNew={create} onRename={rename} onClose={close} onMove={moveTab}
             onFocusTerm={() => window.setTimeout(() => { const id = activeRef.current; if (id) handles.current[id]?.armCtrl(false) }, 0)} />}