fix(ui): theme toggle is now actually reactive

The toggle changed _mode and called setState on StudioAppState,
but the sidebar that hosts the toggle button never rebuilt:
StudioApp returns `MaterialApp(home: const StudioShell())`, and
the const child's element doesn't get a didUpdateWidget on
parent rebuild. So the toggle's icon would only flip on the
next health-poll tick (up to 5s later) when the sidebar's own
setState fired for an unrelated reason.

Switch to a ValueNotifier-based pattern:
- StudioAppState exposes `modeNotifier` (ValueNotifier<...>)
  instead of a getter.
- StudioApp wraps MaterialApp in a ValueListenableBuilder
  bound to the notifier so themeMode flips immediately.
- _ThemeToggle is itself a ValueListenableBuilder against the
  same notifier — its icon and onPressed always reflect the
  current value without depending on parent rebuild timing.

Bumps fai_studio 0.7.2 -> 0.7.3.

Signed-off-by: flemming-it <stefan.a.flemming@googlemail.com>

pubspec.yaml

@@ -1,7 +1,7 @@
 name: fai_studio
 description: "F∆I Studio — desktop GUI for the F∆I hub"
 publish_to: 'none'
-version: 0.7.2
+version: 0.7.3
 
 environment:
   sdk: ^3.11.0-200.1.beta