fix(ui): align dialog typography, spacing, and floating footer

[kev1n77]

Summary

Align shared dialogs with the UI design:

• Use 24px bold headings with a 29px line height and normal letter spacing.
• Make header and footer separators opt-in through the separator prop.
• Replace the opaque floating footer with a gradient and backdrop blur.
• Reserve body and scroll padding so floating actions do not obscure content.
• Remove redundant spacing overrides from the Design Lab preview.

Type and Areas

Type: Bug fix / UI/UX

Areas: Design system, shared UI components, design tokens, Design Lab, generated data-migrator styles.

Motivation / Impact

Dialogs previously used mismatched heading typography, default divider lines, and an opaque floating footer. Content spacing also depended on a Lab-specific override.

This change gives product dialogs and Lab previews consistent typography and spacing, while keeping content accessible beneath floating actions.

Verification

Recorded validation for this patch:

• pnpm run design-system:check — passed, including dialog and text-clipping tests.
• pnpm run check:web — passed, including theme audits and TypeScript checks.
• Tests cover default and explicit separators, floating-footer styling, and public token usage.

These checks were not rerun while drafting this PR description. Full visual verification across themes, narrow windows, and accessibility display modes remains pending.

Reviewer Notes

• Header and footer separators now default to false. Consumers requiring a divider can pass separator.
• Forced-color and reduced-transparency modes use a solid footer fallback.
• Overlay dismissal and focus behavior remain unchanged.
• No backend protocols or persisted data shapes change. Remote scenarios were not exercised.

Checklist

• This PR is focused and does not include secrets, temporary prompts, generated scratch files, or unrelated artifacts.
• Relevant verification is recorded above, or skipped checks are explained.
• User-facing strings, docs, and locales are updated where applicable.