Component catalog demo app

[colemancda]

Replaces the 13-screen gallery with a component catalog: a NavigationStack + lazy List of 21 playgrounds, one per supported SwiftUI feature, each pushed as a navigation destination.

Playgrounds

• Controls — Text, Button, Toggle, Slider, TextField, Picker, ProgressView
• Layout — Stacks, Spacer & Divider, Color, ScrollView
• Containers — List (pull-to-refresh), Grid (fixed/adaptive/horizontal), Modifiers
• Presentation — Navigation, TabView, Sheet, Alert
• State — State, Environment, Observable

Shared Example row and AnyCatalogScreen shim live in CatalogSupport.swift; the old per-screen gallery files are removed.

Verification

Built for Android, deployed, and exercised on the emulator:

• Catalog list renders through the lazy List + NavigationLink path
• Slider playground shows live value readouts
• Modifiers playground renders padding, fixed frame, background, corner radius, offset, and rotation correctly — the transform/layout category the previous renderer could not handle

Dependency

Includes one commit from the still-unmerged feature/desktop-gallery work ("Render the gallery catalog in the desktop rig"); that commit drops out of this diff once desktop-gallery lands.