fix(fortal): restore dialog and popover layout defaults

[leoafarias]

Description

Restores the layout defaults declared by the pinned Radix 3.3.0 parity manifest:

• FortalDialog now defaults to centered placement and a maximum width of 600 logical pixels.
• FortalDialogAlign.start opts into top-centered placement.
• FortalPopover now has a maximum width of 480 logical pixels.
• Generated APIs, parity coverage, widget tests, and documentation now describe and verify the same contract.

Previously, expanding dialog and popover children could produce unconstrained surfaces, and dialog callers had to add their own Center wrapper even though centered placement was the declared default.

Examples

Before, callers centered dialogs manually:

builder: (context) => Center(
  child: FortalDialog(
    title: 'Confirm changes',
    child: const Text('Review the changes before continuing.'),
  ),
);

Now centering and the 600-pixel maximum width are defaults:

builder: (context) => const FortalDialog(
  title: 'Confirm changes',
  child: Text('Review the changes before continuing.'),
);

Top-centered placement is explicit:

builder: (context) => const FortalDialog(
  align: FortalDialogAlign.start,
  title: 'Confirm changes',
);

Popover content is constrained by the Fortal preset even when a child requests more space:

FortalPopover(
  popoverChild: const SizedBox(width: 640),
  child: const Text('Open'),
);

Validation

• fvm flutter analyze --fatal-infos from packages/remix_fortal
• fvm dart run melos run ci --no-select, including clean code generation, docs validation, Fortal parity checks, and all package tests

Related Issues

Closes #128.

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Checklist

Note: Updating the pubspec.yaml and CHANGELOG.md is not required. These are handled automatically during the release process.

• My PR includes unit or integration tests for all changed/updated/fixed behaviors.
• I have updated or added relevant documentation (doc comments with ///).
• I am prepared to follow up on review comments in a timely manner.

Breaking Change

Does this PR require users of the package to manually update their code?

• Yes, this is a breaking change.
• No, this is not a breaking change.