The Accessibility font family setting (Settings > Appearance > Accessibility) currently offers a small set of proportional fonts. Adding a monospace option would meaningfully improve phishing resistance for security-conscious users.

Monospace fonts make homoglyph attacks visible at a glance:
- rn vs m — clearly two characters in monospace, nearly identical in proportional
- l vs 1 vs I — distinct glyphs instead of ambiguous strokes
- Cyrillic а vs Latin a — spacing differences become obvious
- paypa1.com vs paypal.com — the 1 stands out immediately

Feature Request:
Add a "Monospace" option to the existing font family dropdown in Accessibility settings. When selected, apply a monospace font stack to email content rendering (including the message body iframe).

Benefits:
- Anti-phishing: Users can visually catch spoofed domains and lookalike characters in sender addresses, URLs, and email bodies
- Zero new UI surface: Slots into the existing Accessibility font picker — one new option in an existing dropdown
- Aligns with Proton's identity: Proton already markets itself as the security-first email provider. This is a small feature that reinforces that positioning
- Accessibility overlap: Monospace fonts also benefit users with dyslexia or visual processing differences, since every character occupies the same width

Implementation suggestion:
Add "Monospace" as a new entry in the Accessibility font family dropdown. Apply a standard fallback stack ("SF Mono", "Fira Code", "Source Code Pro", "Cascadia Code", "Consolas", "Liberation Mono", monospace) to the message content iframe when selected. The infrastructure for injecting styles into the content iframe already exists (Proton uses it for dark mode).