feat(ui): stretch the UI to the window aspect — support ultrawide properly

Jeroen on a 3440x1440 panel: 'it does not fit the viewport', then 'widescreen
users are fully supported so we add stretching ui'.

client/project.godot declares a 1920x1080 base viewport with
stretch/mode=canvas_items, and stretch/aspect was unset — which defaults to
'keep'. On a 21:9 display Godot letterboxes the entire 16:9 UI inside it, so
every screen in the game was pillarboxed, not just the Atlas. Measured: the
window came out 2432x1368, exactly 16:9; with aspect=expand it becomes
3440x1368, the full panel width.

Worth being explicit that the Atlas was innocent here. It was filling its
logical viewport correctly the whole time; that viewport was being boxed
inside the monitor. The three genuine Atlas sizing bugs fixed today (the
zeroed extent, the legend-column mismatch, the integer-ratio floor) were all
real and all separate from this.

expand gives every screen more space on a wide display instead of bars, which
is the right default for a UI-heavy game with a HUD and implant panels. The
tradeoff is that layouts must tolerate a variable aspect ratio rather than
assuming 16:9 — accepted deliberately, since widescreen is a supported target.

Verified: full client suite 1833 / 1807 passed / 0 failed, cold-parse clean,
parse sweep clean across 226 scripts.

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

client/project.godot

@@ -48,6 +48,7 @@ theme/custom_font="res://assets/fonts/michroma_ui.tres"
 window/size/viewport_width=1920
 window/size/viewport_height=1080
 window/stretch/mode="canvas_items"
+window/stretch/aspect="expand"
 
 [editor_plugins]