Fix screenshot mode for email body (iframes)

[cheack]

This PR fixes an issue where "Screenshot Mode" did not hide or blur text inside email body iframes.

• Added ScreenshotModeMessageExtension (using MessageViewExtension) to inject styles into email body iframes upon rendering.
• Hide the email subject from the window title while screenshot mode is active.

Before

After

[indent-staging]

Issues

2 potential issues found:

• getStyleText() performs synchronous file I/O on every call to applyToDocument. Since the CSS content is static, it could be cached in a module-level variable for better performance.
• applyToDocument accesses doc.head without a null check. If called on a document without a head element (e.g., a third-party iframe), doc.head.appendChild(el) will throw a TypeError.

Summary

This PR extends screenshot mode to properly obscure email content by applying the screenshot-mode CSS to email body iframes (not just the main document) and hiding the email subject from the window title.

• Registers a MessageViewExtension to apply screenshot-mode styles to newly rendered email bodies in iframes
• Patches window.setTitle to show only "Mailspring" (without subject) when screenshot mode is enabled
• Iterates existing iframes on toggle to apply/remove styles from all visible email content
• Properly cleans up extension registration and title patching on deactivate

CI Checks

Waiting for CI checks...

Rule Checks 3 rules evaluated, 3 passed, 0 failed

Passing rules

C
B
Ben Rule

[indent-staging]

Potential null reference: doc.head could be null for documents without a head element (e.g., third-party iframes). While unlikely in practice since email iframes are created with doc.write() which includes a DOCTYPE, adding a guard would make this more robust: if (doc.head) { doc.head.appendChild(el); }

[indent-staging]

Minor performance optimization: This reads the CSS file synchronously on every call, which happens on toggle and for every new message rendered. Consider caching the result in a module-level variable since the CSS content is static.

[bengotow]

I think this will resolve #1044!