Move to the layout REPO_POLICIES.md gives (closes #30)

The Go module moves from backend/ to the root (go.mod, go.sum,
.golangci.yml, cmd/, internal/), the page's source to web/, and nginx's
two files to configs/. backend/ is gone: its LICENSE and .dockerignore
are deleted, and its .gitignore entries, README and scripts are folded
into the root ones. script/build builds the server binary after the
frontend; script/run and script/clean move up from backend/script/;
make test, lint, fmt and fmt-check do what the backend's did. Vite's
root is web/ and it still builds into dist/. go.mod ignores
node_modules/, whose npm packages can ship .go files. script/bootstrap
no longer installs gcc, which only the host Go tests needed.

Model: opus-5-5

web/index.html

@@ -0,0 +1,20 @@
+<!doctype html>
+<html lang="en">
+    <head>
+        <meta charset="UTF-8" />
+        <meta name="viewport" content="width=device-width, initial-scale=1.0" />
+        <title>NetWatch - Network Latency Monitor</title>
+        <link
+            rel="icon"
+            type="image/svg+xml"
+            href="data:image/svg+xml,<svg xmlns='http://www.w3.org/2000/svg' viewBox='0 0 100 100'><text y='.9em' font-size='90'>📡</text></svg>"
+        />
+        <!-- Linked here, not imported by web/src/main.js, so the unit tests can
+             import that module in Node, which cannot import CSS. -->
+        <link rel="stylesheet" href="/src/styles.css" />
+    </head>
+    <body class="bg-gray-900 text-white min-h-screen">
+        <div id="app"></div>
+        <script type="module" src="/src/main.js"></script>
+    </body>
+</html>