docs: add GitBook footer chrome — footer and mobile overlay fix

[Lythaeon]

Description

Adds a minimal footer to the GitBook docs shell and fixes the remaining mobile sidebar behavior so the navigation drawer overlays the page instead of pushing content sideways.

Changes

• docs/gitbook/_layouts/website/page.html: add a small footer with SOF, the current year, and a GitHub icon link; update the client-side docs chrome hook to keep the year current.
• docs/gitbook/styles/website.css: style the footer for desktop and mobile, keep it centered and subdued, and continue hiding the broken font-settings control in the header.
• docs/gitbook/styles/website.css: override HonKit's mobile book-body transform so opening the sidebar no longer shifts the main page.
• No runtime, crate, or architecture slice changes.

For slice-related changes, include:

• Affected slices: none
• Cross-slice communication changes (if any) and why: none
• Migration requirements (if any): none

Motivation

Business motivation:

• The docs shell needed a clean, product-like footer instead of ending abruptly.
• Mobile navigation still had an obvious usability issue where the drawer pushed the page content.

Technical motivation:

• Keep the footer logic local to the docs website layout rather than hardcoding a stale year.
• Explicitly neutralize HonKit's default mobile body transform so the custom slide-over summary behaves as intended.

Alternative approaches considered:

• Text footer links instead of the GitHub icon: rejected because it felt heavier in the current chrome.
• Leaving the upstream mobile transform in place: rejected because it conflicts with the custom overlay drawer behavior.

Scope and impact

• Affected slices: none
• Data/API changes: none
• Backward compatibility: unchanged
• Performance impact: negligible; one small DOM year update during chrome render
• Security impact: none

Testing

• Unit tests
• Integration tests
• Manual verification
• Performance checks (if applicable)
• Security checks (if applicable)

Commands/results:

cd docs/gitbook && npm run build

Result: passes successfully.

Related issues and documentation

• Fixes:
• Related:
• Architecture docs: docs/architecture/README.md
• Relevant ARD/ADR:
• Operations/runbook updates: none

Reviewer checklist

• Code follows project standards and architecture constraints
• Slice boundaries are respected (docs/architecture/ard/0003-slice-dependency-contracts.md)
• Tests added/updated and passing
• Documentation updated (README/docs/operations as needed)
• No undocumented breaking change
• Performance trade-offs documented where relevant
• Security considerations addressed where relevant

Additional notes

• Untracked local files in .github/assets/ were intentionally left out of this PR.