docs: update feature screenshots

[floatpanebot]

What?

Refreshes the feature screenshots in docs/docs/assets/features/.

Screenshots included:

• compose_email.png
• compose_empty.png
• drafts.png
• email_view.png
• inbox_view.png
• main_menu.png
• settings.png
• theme_settings.png
• threading_demo.png

Why?

Keeps documentation visuals aligned with the current TUI. Generated automatically by the Generate Screenshots workflow on the latest release.

[floatpanebot]

❌ Integration failed

Commit: 072de66 · run logs

Failed jobs:

• cross-platform build matrix (ubuntu-latest, 1.26.3) — Run go build ./... and go test ./... locally.
• cross-platform build matrix (macos-latest, 1.26.3) — Run go build ./... and go test ./... locally.
• cross-platform build matrix (windows-latest, 1.26.3) — Run go build ./... and go test ./... locally.