feat: retune the type scale and add title/label emphasis

[rahulbisht25]

Summary

Retunes FlowTypography.standard to the design file and adds emphasised cuts for title and label, then points existing controls at those roles instead of ad-hoc copyWith weights.

• Display / headline / title / label sizes and line heights match the design: display at 1.15, headline at 1.2, title and label at 1.3.
• New public styles: titleLargeEmphasised, titleMediumEmphasised, titleSmallEmphasised, labelLargeEmphasised, labelMediumEmphasised, labelSmallEmphasised.
• Compact FlowGreeting uses titleMedium instead of a one-off 21px override.
• Menus, sheets, pills, suggestions, model selector, error state, and attachment chrome consume the new roles.

Breaking: constructing FlowTypography(...) now requires the new emphasised styles. FlowTypography.standard and copyWith cover the usual host paths. Label sizes also step up one rung (16 / 14 / 12).

Screenshots

Visual change is type size/weight across greeting, menus, pills, suggestions, and error chrome. Screenshots from the playground stages to follow.

Before	After

How this was verified

• Playground stages that consume the retuned roles: greeting, menus / model selector, pill, suggestions, error state, attachments.
• dart format on the changed Dart files.

Checklist

• flutter analyze lib and flutter analyze in example/ and playground/ are clean
• dart format . applied
• Exercised in the playground — with a stage demo added or updated if this is a new component or variant
• No new entries under dependencies: in pubspec.yaml (Flutter SDK and flutter.dev packages only)
• Nothing model-facing — no prompts, schemas, or provider/network calls
• New public API is exported from lib/flow_ui.dart and documented in docs/ and the README table
• CHANGELOG.md updated for user-facing changes, with breaking changes called out
• PR title follows conventional commits (feat:, fix:, refactor:, docs:, chore:)

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