feat(web): SVG logo + favicon, replace corrupted PNGs

[themightychris]

Summary

Two related fixes you flagged after #123 landed:

1. The PNGs from fix(web): always-visible wordmark in navbar #123 were corrupted. The bytes I wrote via base64 from the Drive download were off by 300+ bytes from the original metadata — the decoded files were valid PNGs with reasonable dimensions, but the wrong content. Replaced all three with the correct binaries.
2. Added SVG variants for the two surfaces that actually render the logo — the navbar (`/img/logo-horizontal.svg`) and the browser tab favicon (`/favicon.svg`). SVG scales perfectly at any zoom, scales for retina without serving a 2x asset, and is roughly half the payload of the new high-res PNG.

Updates:

• `AppHeader.tsx` swaps `/img/logo-horizontal.png` → `/img/logo-horizontal.svg`.
• `index.html` prefers the SVG favicon via `<link rel="icon" type="image/svg+xml">` with the PNG retained as a fallback for old browsers (pre-Chrome 80 / pre-Firefox 41).
• `logo.png` (square standalone) is updated to the correct binary but still isn't referenced anywhere — kept on hand for future use.

Test plan

• `npm run -w apps/web type-check` clean
• `npm run -w apps/web test -- tests/AppHeader.test.tsx` — 5/5 (header test asserts alt-text + link target, not the specific src)
• `npm run lint` clean
• After deploy: confirm logo renders crisply in the navbar + favicon appears in the tab on https://next-v2.codeforphilly.org

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