Add an SVG favicon (no more /favicon.ico 404)

Link a small inline SVG (site/favicon.svg, a red tile with a white "V") from
both index.html and board.html via a root-absolute href, so it resolves from
"/" and from "/board/" under Vercel cleanUrls. With an icon link present,
browsers stop auto-requesting (and 404ing on) /favicon.ico. No binary asset and
no external dependency, consistent with the self-contained client.

e2e asserts both pages declare the favicon and that /favicon.svg is served as
valid SVG. Closes the last code-finishable item in the roadmap's smaller list.

Co-Authored-By: Claude Opus 4.8 <[email protected]>

docs/nostr-static-client-spec.md

@@ -28,6 +28,7 @@ site/
   locales.js
   board.html          (bulletin board page)
   board.js
+  favicon.svg         (inline SVG site icon)
   CNAME
   vendor/
     nostr-tools/

docs/roadmap.md

@@ -240,7 +240,8 @@ should follow the board's trust model.
   varies with font availability and zoom; the
   [cheat sheet](flyer-field-limits.md) is explicitly approximate. Fine as-is;
   noted so it is not mistaken for a contract.
-- **No favicon** — browsers log a harmless `404 /favicon.ico`. Cosmetic.
+- **Favicon — ✅ done.** A small inline SVG (`site/favicon.svg`) is linked from
+  both pages, so browsers no longer request (and 404 on) `/favicon.ico`.
 - **WebKit/Safari coverage in CI** needs `libavif16` on Linux runners
   (`sudo npx playwright install-deps`); headless WebKit is also not a perfect
   stand-in for real Safari/iOS print + clipboard behavior, so a real-device