Slim the shared site footer

[shaug]

TL;DR

Collapse every web footer into one responsive navy band with concise, linked data and site provenance.

Summary

• centralize election-footer provenance for Guide, Sources, and Compare while giving About, archive, and 404 the global variant
• keep panel identity on election pages, remove election wording, and link data/code hashes to their artifacts
• move provenance beside the brand on wide screens and to one full-width squeeze row without displacing icon targets
• preserve the existing print audit grammar and update design-ledger decisions
• cover 900px, 390px, and 320px layout behavior plus worker-rendered 404 provenance

Validation

• make check
• fresh final full-diff review: clean, no findings

Fixes #179