feat(intro): wire flutter_animate effects + confetti (v1.2)

[code418]

Summary

Ships the v1.2 Intro polish ship gate from docs/plans/ROADMAP.md:

• Postbox fadeIn + elastic scale on the opening step.
• Postman James slideX with easeOutBack bounce on step 1, replacing the hand-rolled AnimationController + FractionalTranslation.
• "Points, baby!" shimmer on the Mega Points step, with a gold/red/white confetti burst fired from _advance as the user enters.

Adds flutter_animate: ^4.5.0 and drops TickerProviderStateMixin from _IntroState now that flutter_animate owns its own tickers. Keeps the _scrollCentre landscape-overflow fix and the _completed double-tap guard intact.

Supersedes #82

PR #82 was dependency-only on a branch that had drifted off master far enough to clobber bloc 9, firebase_messaging, firebase_remote_config, the landscape-overflow fixes, and several assets. This PR bundles the dependency add and the wiring follow-up the roadmap called for, against current master. Close #82 once this merges.

Ship gate

• flutter analyze clean (2 pre-existing infos in postbox_marker_test.dart, unrelated)
• flutter test — 340 / 340 passing
• Effects wired into lib/intro.dart

Test plan

• Replay intro from Settings → ▶ "Watch intro again". Confirm step 0 postbox fades + scales in, step 1 James slides in with bounce, step 4 text shimmers and confetti fires once on entry.
• Confirm no animation regression on landscape (_scrollCentre preserved).
• Double-tap "Get started" rapidly — should still only fire onDone once (the _completed guard).

🤖 Generated with Claude Code