give the welcome screen glass chrome and shadcn polish

- Wrap the pairing action dock in a rounded glassEffect container with linear-gradient edge fades at the top and bottom of the scroll, producing the native iOS 26 Liquid Glass chrome without masking blur hacks.
- Realign the welcome layout to match the shadcn cards used by the home tab: tighter 40pt hero glyph, ShadcnBadge callout, AppSectionCard step/note rows, and PrimaryActionStyle/SecondaryActionStyle buttons instead of a tint-colored glassProminent pill.
- Add a VariableBlurView utility in GlassChrome.swift (adapted from nikstar/VariableBlur, MIT) for cases where a real progressive Gaussian blur is needed, and capture the chrome/blur playbook plus tsswift screenshot workflow notes in readme.knowledge.md.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

swift/Sources/Core/Design/ButtonStyles.swift

@@ -15,7 +15,7 @@ struct PrimaryActionStyle: ButtonStyle {
                 .frame(maxWidth: .infinity)
                 .padding(.horizontal, 18)
                 .frame(height: 44)
-                .foregroundStyle(Color.idpPrimaryForeground)
+                .foregroundStyle(isEnabled ? Color.idpPrimaryForeground : Color.idpMutedForeground)
                 .background(
                     RoundedRectangle(cornerRadius: IdP.controlRadius, style: .continuous)
                         .fill(isEnabled ? Color.idpPrimary : Color.idpMuted)