feat(theme): wallpaper-derived dynamic accent for buttons

[bmc08gt]

What

Adds rememberDynamicAccent() to :ui:theme — a small helper that returns the user's Material You (Monet) accent color derived from their wallpaper / system theme, à la Google Photos' Save action.

On Android 12+ it reads the palette via Material3's dynamicDarkColorScheme() (we're dark-only, so we always take the dark scheme) and returns its primary / onPrimary. Below API 31 it falls back to the brand colors you pass in.

val (accent, onAccent) = rememberDynamicAccent(
    fallbackAccent = CodeTheme.colors.secondary,
    fallbackOnAccent = CodeTheme.colors.onAction,
)
Button(colors = ButtonDefaults.buttonColors(containerColor = accent)) {
    Text("Save", color = onAccent)
}

First use: the currency creator's bill-customization action button (relabelled "Done" → "Next") now uses the system accent instead of a flat white pill.

Notes

• Pulls compose-material3 into :ui:theme for the first time. This is the intended first step toward migrating the theme layer off the bespoke Material2-based color system onto Material3 — the natural follow-up is mapping the full dynamicDarkColorScheme into com.getcode.theme.ColorScheme at the DesignSystem(colorScheme = …) entry point so dynamic color can flow app-wide.
• minSdk is 29, so the helper guards on Build.VERSION.SDK_INT >= S and falls back to brand colors on older devices.
• No new manifest permissions required.

Testing

• :ui:theme:compileDebugKotlin and :apps:flipcash:features:currency-creator:compileDebugKotlin both build clean (verified in an isolated worktree at this commit).