fix(studio): sidebar — single AnimationController drives width

+ label opacity in lockstep (no more glitchy expand)

The previous "fixed-width icon column" commit already locked
the icon's horizontal pixel-X, but the rail still _felt_
glitchy on expand because the rail's geometry and the rail's
content were animated by two different mechanisms:

 - Width: AnimatedContainer over FaiMotion.fast (120 ms)
 - Content: setState(_hovered = true) → conditional
   `if (expanded) Expanded(label)` snaps in one frame

So on mouse-enter the label widget appeared INSTANTLY while
the rail was still 72 px wide. The label tried to render in
0 px of available space and Flutter's layout engine clamped
it; over the next 7 frames the rail grew to 220 px and the
label visibly "settled in". That's the perceived glitch.

Replace the two-source animation with a single
SingleTickerProviderStateMixin + AnimationController whose
value `t` (0..1, eased via easeInOutCubic) drives both:

 - rail width = lerp(72, 220, t)
 - label opacity = t
 - labelsInteractive = t > 0.5 (so hidden buttons can't
   eat clicks meant for the icon column)

Labels are wrapped in `t > 0 ? IgnorePointer(Opacity(...))
: SizedBox.shrink()`. Once the animation starts, the label
joins the tree at opacity ≈ 0 (invisible — no pop) and
fades up smoothly as t grows. When fully collapsed (t == 0)
the label is removed from layout entirely, so the connection
pill's tall "Start hub" affordance doesn't inflate the
collapsed rail's height (this also fixes the widget_test
vertical-overflow that 0.51.4 introduced).

Same pattern applied to the footer (settings icon stays in
the 72-px column always; theme/lang/clock fade in beside it)
and to each _SidebarItem destination row.

End result: width AND content travel together along the same
animation curve. No more "snap then catch up" — they're
mathematically inseparable.

Version 0.51.4 → 0.51.5.

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: 0.51.4
+version: 0.51.5
 
 environment:
   sdk: ^3.11.0-200.1.beta