Add DocumentMode groundwork with MaskMode and Q toggle

[krVatsal]

This PR implements the early groundwork for Document Mode as planned for community bonding / week 1 in Marquee Selection Masking.
It introduces MaskMode and wires a keyboard toggle, while intentionally avoiding rendering or visual feature work.

What this PR changes?

• Re-enables DocumentMode enum and adds MaskMode
• Stores current document_mode in editor state
• Adds ToggleDocumentMode message
• Handles toggle logic in document message handler:

1. DesignMode -> MaskMode
   

2. MaskMode -> DesignMode
   

• Maps Q to ToggleDocumentMode

Scope and intent

• Pure editor state change only
• No rendering pipeline changes
• No frontend UI additions
• No behavior beyond mode state toggle

[cubic-dev-ai]

No issues found across 4 files

Confidence score: 5/5

• Automated review surfaced no issues in the provided summaries.
• No files require special attention.

[gemini-code-assist]

Code Review

This pull request introduces a new DocumentMode enum, including a MaskMode variant, and integrates it into the DocumentMessageHandler to allow toggling and setting the document mode. The review feedback suggests ensuring UI consistency by triggering PortfolioMessage::UpdateDocumentWidgets when the mode changes and recommends uncommenting and updating the fmt::Display implementation for DocumentMode to avoid leaving dead code.