feat(editor): add, edit and remove PDF link annotations

[McanKul]

Goal

Allow users to manage standard URI and internal-page links in the visual editor while preserving unrelated PDF content.

Scope

• Detect existing URI and GoTo link annotations.
• Show link regions at the correct page position.
• Create, edit and remove supported links.
• Preserve unsupported actions and unrelated annotations unchanged.

Out of scope

• JavaScript, Launch, file, or remote-GoTo actions.
• Automatically opening links from the editor.

Acceptance criteria

• Placement is correct for CropBox/MediaBox and all page rotations.
• URI and internal-page links reopen correctly in at least two common PDF viewers.
• Unsafe schemes and unsupported actions are rejected with a clear message.
• A no-op save preserves existing annotations and document structure.
• Original files are never overwritten.
• qpdf --check and automated rotated, cropped, and malformed fixtures pass.

Related work

• Part of Epic: Offline visual PDF editing #12.
• Shares annotation preservation and appearance work with Add comments, highlights and drawing annotations #8.
• Uses the output validation gate tracked separately in this milestone.