refactor(web): one source for the canonical origin, and document the move

vite.config.ts and src/config.ts each held their own copy of
https://annotkit.gpu-cli.sh. They are read in different build contexts —
Node at build time, the browser at runtime — so nothing forced them to
agree, and changing one without the other would ship a canonical tag and
an OG url pointing at different hosts. Both now import shared/site.ts.

Also documents the domain move, which was only half-covered: the README
listed VITE_SITE_URL in two config tables but never said what a domain
change actually costs. It is three edits — the shared constant, the
footer text in scripts/og-template.html (baked into og.png, so re-run
npm run og), and the two README backlinks.

Notes the honest non-answer too: package.json's description and the
request URLs in tests/subscribe.test.ts mention the domain and neither
matters, so nobody has to wonder whether they do.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

Author: angusbezzina

web/README.md

@@ -189,6 +189,30 @@ at `<branch>.annotkit.pages.dev`.
 Secrets are not part of a deploy. `wrangler pages secret put` writes them once
 per environment (see above), and they persist across deployments.
 
+## Moving to a different domain
+
+Nothing in the build needs to change to *launch* on `annotkit.gpu-cli.sh` —
+that is already the default, so the canonical and OpenGraph tags point at the
+final domain even while the site is served from `*.pages.dev`. That is
+deliberate: it stops the pages.dev URL being indexed as a duplicate.
+
+Changing to a **different** domain later is three edits and a re-render:
+
+1. `shared/site.ts` — `DEFAULT_SITE_URL`. One constant, imported by both
+   `vite.config.ts` (which substitutes `%SITE_URL%` into index.html) and
+   `src/config.ts` (which hands it to the components). They used to hold a
+   copy each, which is exactly how a canonical tag and an OG url end up
+   disagreeing.
+2. `scripts/og-template.html` — the domain is printed as footer text on the
+   card. Change it, then `npm run og` to re-render `public/og.png`.
+3. The root `README.md` backlink, and this file's first line.
+
+Then add the custom domain to the Pages project and point a CNAME at it.
+
+`web/package.json`'s `description` and the request URLs in
+`tests/subscribe.test.ts` also mention the domain; neither is load-bearing
+(the tests only need a well-formed absolute URL to build a `Request`).
+
 ## Design
 
 Hallmark, genre **editorial**: macrostructure **Specimen**, theme **Specimen**,

web/shared/site.ts

@@ -0,0 +1,17 @@
+/**
+ * The canonical origin, in one place.
+ *
+ * Two build contexts need it and they cannot see each other's globals:
+ * `vite.config.ts` substitutes it into index.html at build time (Node), and
+ * `src/config.ts` hands it to the components (browser). Holding a copy in
+ * each is how the canonical tag and the OG url end up disagreeing after a
+ * domain change, so both import this.
+ *
+ * `VITE_SITE_URL` overrides it per environment; this is only the fallback.
+ *
+ * NOT the only place the domain appears — `scripts/og-template.html` prints
+ * it as footer text, and that is baked into `public/og.png`. Changing the
+ * domain means changing that too and re-running `npm run og`. See the
+ * "Moving to a different domain" section of README.md.
+ */
+export const DEFAULT_SITE_URL = "https://annotkit.gpu-cli.sh";

web/src/config.ts

@@ -6,10 +6,11 @@
  * should be a one-file change, not a grep.
  */
 
+import { DEFAULT_SITE_URL } from "../shared/site";
+
 export const site = {
   name: "AnnotKit",
-  /** STUB — confirm before launch (epic §11, maintainer action 6). */
-  url: import.meta.env.VITE_SITE_URL ?? "https://annotkit.gpu-cli.sh",
+  url: import.meta.env.VITE_SITE_URL ?? DEFAULT_SITE_URL,
   repo: "https://github.com/gpu-cli/annotkit",
   issues: "https://github.com/gpu-cli/annotkit/issues",
   license: "https://github.com/gpu-cli/annotkit/blob/main/LICENSE",

web/vite.config.ts

@@ -1,7 +1,7 @@
 import { defineConfig, type Plugin } from "vite";
 import react from "@vitejs/plugin-react";
 
-const DEFAULT_SITE_URL = "https://annotkit.gpu-cli.sh";
+import { DEFAULT_SITE_URL } from "./shared/site";
 
 /**
  * Substitutes `%SITE_URL%` in index.html so the canonical and OpenGraph URLs