fix(release): keep git tree clean for goreleaser

The dist copy deleted tracked .keep and overwrote tracked index.html —
dirty tree aborted the release. index.html is now untracked (bare clones
serve 404 instead of a stale stub), .keep alone satisfies go:embed, and
the workflow merges into dist/ without rm. fetch-depth: 0 for tag history.

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Co-Authored-By: Devin <158243242+devin-ai-integration[bot]@users.noreply.github.com>

apps/backend/internal/web/dist/index.html

@@ -1,21 +0,0 @@
-<!doctype html>
-<html lang="en" data-theme="dark">
-  <head>
-    <meta charset="UTF-8" />
-    <meta name="viewport" content="width=device-width, initial-scale=1.0" />
-    <title>Dash</title>
-    <script>
-      // Pre-paint theme: localStorage mirrors the server setting, applied
-      // before first paint so there is no flash of the wrong theme.
-      try {
-        var t = localStorage.getItem("dash.theme");
-        document.documentElement.dataset.theme = t === "light" ? "light" : "dark";
-      } catch (e) {}
-    </script>
-    <script type="module" crossorigin src="/assets/index-WK6y87Wv.js"></script>
-    <link rel="stylesheet" crossorigin href="/assets/index-CHTjdWqd.css">
-  </head>
-  <body>
-    <div id="root"></div>
-  </body>
-</html>