Refresh demo UI and visual QA

[RockerFlower]

Context

The example app demonstrates the package but still has an older Flutter demo shape. The maintenance PR keeps UI changes small and limited to analyzer cleanup so the bugfix remains low risk.

Goal

Refresh the example/demo experience so it better presents the wave package while preserving the package API and avoiding unrelated breaking changes.

Suggested scope

• Audit the current example app layout, colors, typography, and responsive behavior.
• Keep the demo focused on real WaveWidget configurations rather than marketing copy.
• Add examples for CustomConfig, SingleConfig, RandomConfig, gradients, background images, blur, and non-looping animation.
• Verify desktop and mobile web layouts visually.
• Avoid introducing heavy dependencies unless they clearly improve the demo.

Acceptance criteria

• The example builds and runs on the selected Flutter SDK.
• flutter analyze passes.
• Key wave modes are visible in the first screen or an obvious demo list.
• Desktop and mobile screenshots show no overflow or broken layout.
• README links or screenshots are updated if the demo presentation changes.