Settings › Traffic speaks the home page's language #9775

Part of the design pass that already covered Settings › Account, Passkeys, Backups and Capacity. The owner reads this page to see who comes to their forge and from where, so it should read as calmly as the home page.

Done looks like:

• [x] /settings/traffic (src/web/pages/traffic.ts) follows the principles that open docs/DESIGN.md, as the Settings › Account pass (eb8dfff06) did: one heading, numbers before words, no more cards or rules than the content needs, and the same type scale and spacing as the home page.
• [x] Everything the page shows today is still there (referrers, campaigns, sources, history), and test/traffic-history.test.ts, test/traffic-sources.test.ts and test/traffic-truth.test.ts pass unchanged. If one has to change, the pull says why.
• [x] A warm request still answers under 50 ms, and the page works with JavaScript off.
• [x] bun scripts/design-lint.ts passes. Any new class goes in docs/design/areas/settings.md, not docs/DESIGN.md.

Files: src/web/pages/traffic.ts, and src/web/pages/sources.ts only if it renders part of this page. Risks: referrer and campaign strings come from strangers, so every one stays escaped. Change no counting, only how the counts are shown.

Reviewed pull #9844 at its head: ticked /settings/traffic (src/web/pages/traffic.ts) follows the principles that open docs/DESIGN.md, as the Settings › Account pass (eb8dfff06) did: one heading, numbers before words, no more cards or rules than the content needs, and the same type scale and spacing as the home page.; Everything the page shows today is still there (referrers, campaigns, sources, history), and test/traffic-history.test.ts, test/traffic-sources.test.ts and test/traffic-truth.test.ts pass unchanged. If one has to change, the pull says why.; A warm request still answers under 50 ms, and the page works with JavaScript off.; bun scripts/design-lint.ts passes. Any new class goes in docs/design/areas/settings.md, not docs/DESIGN.md..