fix(web): align provider dialogs and add workspace composition coverage

[kev1n77]

Summary

• Set the shared xl dialog maximum width to 800px.
• Use a floating footer in the provider editor and let DialogBody own scrolling instead of nesting another scroll container.
• Update the Design Lab provider preview and add an interactive workspace form with sample paths.
• Add workspace-dialog regression tests for creation, failure recovery, and peer-aware directory selection.

Type and Areas

Type: Bug fix / UI/UX / test

Areas: Web UI, design tokens, Design Lab, shared UI documentation, generated data-migrator styles.

Motivation / Impact

Provider dialogs previously used a narrower layout and nested scrolling that competed with the shared dialog's spacing and footer behavior.

The updated composition provides consistent width, scrolling, and floating actions. Workspace creation retains its small dialog, attached footer, existing control sizes, and internal field dividers.

Verification

Recorded validation for this patch:

• pnpm run design-system:check — passed.
• pnpm run check:web — passed, including theme audits and TypeScript checks.
• Focused product tests — 3 files, 10 tests passed.
• Reviewed the supplied workspace-dialog screenshot against the intended static appearance.

Workspace tests cover disabled controls during creation, form preservation after failure, peer-aware picker delegation, and remote-style path preservation.

Checks were not rerun while drafting this description. Full provider scrolling, keyboard, theme, and narrow-window visual verification remains pending.

Reviewer Notes

• The xl width change affects all consumers of that shared dialog size.
• The workspace dialog's existing fill/primary button variants remain unchanged; this PR adds regression coverage.
• Lab directory choices use local sample data and do not call product APIs.
• Remote picker tests use mocks; no remote end-to-end scenarios were exercised.
• No backend protocols or persisted data shapes change.

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.