fix: surface real network cause on Gitea issue creation failure (v0.51.4)

A thrown fetch error (DNS failure, connection refused, timeout) never
reached the res.ok branch, so its actual cause was lost — err.message
alone is often just "fetch failed" with the real reason nested in
err.cause. Now logs the full cause server-side and folds it into the
error returned to the admin support view's tooltip.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

package.json

@@ -1,6 +1,6 @@
 {
   "name": "epicure",
-  "version": "0.51.3",
+  "version": "0.51.4",
   "private": true,
   "scripts": {
     "dev": "pnpm --filter web dev",