feat: Tier 1 intro polish — flutter_animate

[code418]

What

Adds flutter_animate as a dependency.

Note: confetti (^0.8.0) is already in pubspec.yaml from a previous addition.

Why

flutter_animate provides chainable, composable animation effects (fadeIn, slideX, shimmer, bounce, scale) on any widget with near-zero boilerplate. Intended for use on the intro screen:

// Postbox fades + scales in on step 0:
SvgPicture.asset('assets/postbox.svg', width: 120)
    .animate()
    .fadeIn(duration: 600.ms, curve: Curves.easeOut)
    .scale(begin: const Offset(0.6, 0.6), duration: 600.ms, curve: Curves.elasticOut)

// James slides in with bounce:
PostmanJamesSvg(size: 100)
    .animate()
    .slideX(begin: -1.5, end: 0, duration: 900.ms, curve: Curves.easeOutBack)

// "Mega points!" text shimmers:
Text('Mega points!')
    .animate(onPlay: (c) => c.repeat())
    .shimmer(duration: 1200.ms, color: postalGold)

confetti can be wired to the Mega Points step with gold + red colours.

Not wired up yet

Dependencies only. Follow-up PR wires the effects into lib/intro.dart.

Test plan

• flutter pub get succeeds
• flutter analyze clean
• No runtime regressions on intro or home screen