fix(studio): sidebar — fixed-width icon column eliminates icon shift

Previous "left-anchor everything via padding" approach made
the math work on paper but the perceived shift persisted. The
root cause was structural, not padding-based: ListView gave
each item a tight cross-axis constraint equal to the rail's
current width (48 collapsed, 196 expanded). Inside the item,
the AnimatedContainer filled that constraint and laid out a
mainAxisSize.min Row at the start of the AC's content area.
The icon's pixel-X depended on the AC's padding _and_ the
constraint; even with equal padding the rendered layout
shifted across the expand animation because Flutter
re-resolved alignment under a moving constraint.

Replace the entire pattern with a fixed-width icon column
that doesn't care about the parent constraint:

  ListView (padding 0) → Item Row
    SizedBox(width: 72) → Center → icon   ← anchored to literal x
    if expanded → Expanded → label text   ← grows into remaining space

Same pattern for the brand/connection/channel rows above. The
72 px matches the rail's collapsed width exactly, so collapsed
rail = one icon column + nothing; expanded rail = same icon
column + label slot. Icon's pixel-X is now provably constant.

Removes the AC's horizontal padding entirely — the icon
column owns the left anchor. The background highlight still
spans the full row width (typical nav-rail UX), and the rail
expand animation only moves the rail's right edge, never any
icon-bearing element.

Also passes an `iconColumnWidth: 72` to _SidebarItem instead
of hard-coding the magic number in two places.

Version 0.51.3 → 0.51.4.

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: 0.51.3
+version: 0.51.4
 
 environment:
   sdk: ^3.11.0-200.1.beta