Add accessible skip-to-main-content links

[jpagh]

Summary

Adds accessible “Skip to main content” links across docassemble pages and interviews.

Changes

• Added the translated “Skip to main content” phrase.
• Added skip links to the shared base template and interview pages.
• Ensured links are the first keyboard Tab stop.
• Added role="main" and tabindex="-1" to main content targets.
• Preserved the interview skip link after AJAX navigation.

Verification

• Confirmed the first Tab focuses the skip link.
• Confirmed activating it moves focus to the main content.
• Verified standard pages and interview pages.
• Verified the link remains available after interview navigation.

Issue SuffolkLITLab#36

Edit: This requires more discussion regarding the correct handling of both the skip to content link and pre-selected fields.