Rename masthead nav label About → How this works

[shaug]

Follow-up from the #149 closeout review · Rules: docs/DESIGN.md § Text vs. icons, § Titles & naming

DECIDED 2026-07-31 (Scott): the masthead nav label "About" becomes
"How this works" — confirming the companion left optional in
docs/UI_POLISH_ROUND5.md M71.

Outcome

The primary path to the methodology carries its strongest scent: the masthead
on every page reads "Endorsements · Sources · How this works". With the
footer's methodology entry now icon-only (ⓘ, #166), the masthead text path is
the one DESIGN.md's icon rule leans on — it should say what the page does,
not "About".

Scope

• Rename the nav label in the shared band (site_band_html, shell.py) on
  every page; aria-current behavior unchanged.
• The footer ⓘ keeps aria-label/title "How this works" (already does).
• Keep the About page's <title> page-name as "About" (bookmark/search
  familiarity; the M60 grammar governs structure, not this label) — revisit
  only if it feels off in the tab.
• Page h1 ("About this guide, and how to check our work") unchanged.
• Update the docs/UI_POLISH_ROUND5.md M71 entry (optional companion →
  decided) in the same PR, per the DESIGN.md change protocol.

Non-goals

Footer icon cluster, About page content, URL (/about/ stays).

[shaug]

Delivered by #186 and represented on main by merge commit 8d3514698425822c5e06f08d3fb1a80d2db6a690. Shared masthead coverage, the full repository gate, exact-head CI, and clean repository review passed.