fix(perry-ui-ios): finish #1107 — iOS 26 device rendering of Buttons + nested stack bg

[proggeramlug]

Summary

Finishes the work started in #1109 for issue #1107 (partial-alpha NSColor renders zero glyphs on iOS 26 device). Three regressions / blind spots from #1109's first take remained, all reproduced on a physical iPhone running iOS 26.5:

1. Text + textSetColor(α < 1.0) still painted nothing. fix(perry-ui-ios): #1107 partial-alpha text invisible on iOS 26 #1109 reused the label's borrowed view.font pointer and the un-retained initWithString:attributes: result. The working AttributedText path builds a fresh [UIFont systemFontOfSize:] from the label's current point size and retains the resulting NSAttributedString — this commit mirrors that exact pattern (apply_label_color_via_attributed).
2. Bare Button("x", cb) + widgetSetBackgroundColor(...) rendered invisible. buttonWithType:UIButtonTypeSystem on iOS 26 routes through the Liquid Glass renderer, which overrides explicit setTitleColor: / setBackgroundColor: with the system tint. Switch to UIButtonTypeCustom, seed [UIColor labelColor] as the default title color so a bare button is still visible, and drop the setAttributedTitle:nil cleanup branch from fix(perry-ui-ios): #1107 partial-alpha text invisible on iOS 26 #1109 (on iOS 26 it nukes the button's plain-title rendering along with the attributed state, leaving even α==1.0 buttons blank).
3. widgetSetBackgroundColor on a NESTED UIStackView painted transparent. UIStackView.backgroundColor is documented iOS 14+ but the property doesn't drive painting on inner stacks under iOS 26's compositor. The CALayer's own backgroundColor (CGColor) does paint reliably, so we set both — view-level for non-buggy paths (root stack, iOS 17 sim) and layer-level as the belt-and-suspenders fix.

Related: #1122 (follow-up audit of remaining iOS 26 regressions).

Repro / verification

Bisect of the Wishare Onboarding screen on iPhone running iOS 26.5:

• Before this PR: Text(slogan) with α=0.87 → invisible; Button("Mitmachen") with red bg → invisible; white-bg nested card VStack → transparent over outer pink screen.
• After this PR (with inline literal color objects in the screen file): slogan renders, red button renders, white card paints over pink background. (See screenshots in iOS 26: PR #1109 incomplete — plain Text/Button + VStack/Button backgroundColor still don't render on device #1122.)

Note: a separate Perry TS-compiler issue affects cross-module imports of {r,g,b,a} object literals — those still produce wrong values at textSetColor/widgetSetBackgroundColor call sites even with this PR applied. I'm filing that as a separate issue; this PR only fixes the iOS 26 native-side rendering bugs.

Test plan

• Confirm bare Button with no explicit textSetColor still has visible text on light + dark mode (uses [UIColor labelColor] default)
• Confirm Button with widgetSetBackgroundColor(α=1.0) + textSetColor(WHITE α=1.0) paints red bg + white title on iOS 26 device
• Confirm Text with textSetColor(α=0.87) paints glyphs on iOS 26 device
• Confirm nested VStack + widgetSetBackgroundColor(WHITE α=1.0) paints over a colored parent on iOS 26 device
• Confirm iOS 17 simulator paths unchanged (root stack + bare buttons still render the same way)
• CI green