Refine Material group cards, form fields, and app copy

[seenu-k]

Group artwork competed with group names, while outlined floating labels crossed the edge of filled form fields. This refresh gives group identity more prominence and keeps labels inside a consistent Material filled-field layout.

Changes

• Use filled Material cards with larger, wrapping group names and avatars. Keep the existing cover, name and avatar transitions, with six subdued artwork accents per cover.
• Share field styling through the Material input and dropdown themes across login, New Group and other forms. Preserve the amount field's existing appearance, and let New Group scroll with large text and the keyboard open.
• Shorten explanatory copy and use the landing-page tagline, “Split expenses with friends, free forever.”, across login, About and website branding.
• Add regression coverage, optional native screenshot review output, and an Unreleased changelog entry.

Validation

• 185 presentation and website metadata tests passed.
• Dart analysis with --fatal-infos, formatting and git diff --check passed.
• Release-note and Play metadata tests passed.
• Native light/dark renders reviewed for groups, expense entry, login and New Group. Widget tests cover 200% text scaling, keyboard layout and shared transitions in both directions.

Deployment

UI, copy, tests and screenshot tooling only. No backend, API, dependency, database schema or migration changes; the existing expense description remains a single field.

Merging into main triggers the existing CI and release workflow. A successful release deploys the web bundle with the unchanged Worker and distributes Android to Play closed testing. No additional backend setup is needed for this change.