fix: actually generate the UTF-8 locale makensis needs

`locale-gen <name>` is an Ubuntu extension; plain Debian's locale-gen
takes no arguments and only reads /etc/locale.gen, so the previous fix
generated nothing and makensis kept aborting with exit 84 / "FATAL: main
argv conversion failed!" under the runner's C/POSIX locale.

Write the entry into /etc/locale.gen, run locale-gen with no arguments,
and assert `locale -a` lists it — a missing locale now fails the install
step with a clear message instead of dying four minutes later inside
electron-builder.

build-release.ps1 sets LC_ALL/LANG itself before packaging a Windows
target on a Linux host, picking a real UTF-8 locale out of `locale -a`.
The electron-builder child then inherits it however the script was
started, including a runner that drops step env.

Also adds a non-fatal probe step that runs the cached makensis with
-VERSION under en_US.UTF-8 and under C, so a failure that is *not* the
locale is distinguishable from one that is without another blind fix.

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

.gitea/workflows/release.yml

@@ -63,9 +63,22 @@ jobs:
           sudo dpkg --add-architecture i386
           sudo apt-get update
           sudo apt-get install -y jq wine locales
-          # C.UTF-8 is glibc >=2.35 only (Debian 12+); en_US.UTF-8 works on any
-          # Debian version and just needs generating once.
-          sudo locale-gen en_US.UTF-8
+          # makensis converts its argv through the locale charset and aborts with
+          # "FATAL: main argv conversion failed!" under C/POSIX. C.UTF-8 is a glibc
+          # built-in only from 2.35 (Debian 12+), so en_US.UTF-8 is generated instead.
+          #
+          # `locale-gen <name>` is an Ubuntu extension: plain Debian's locale-gen
+          # takes no arguments and only reads /etc/locale.gen, so the entry goes in
+          # there first. The anchored grep does not match the line the package ships
+          # commented out.
+          grep -q '^en_US.UTF-8 UTF-8' /etc/locale.gen \
+            || echo 'en_US.UTF-8 UTF-8' | sudo tee -a /etc/locale.gen
+          sudo locale-gen
+          locale -a
+          locale -a | grep -qiE '^en_US\.utf-?8$' || {
+            echo "en_US.UTF-8 was not generated; makensis will abort on argv conversion" >&2
+            exit 1
+          }
 
       - name: Ensure PowerShell
         # Debian ships no snapd by default, so this pulls Microsoft's portable
@@ -82,6 +95,25 @@ jobs:
           sudo ln -sf /opt/microsoft/powershell/7/pwsh /usr/bin/pwsh
           sudo chmod +x /opt/microsoft/powershell/7/pwsh
 
+      - name: makensis locale probe
+        # Diagnostic only, never fails the job. If `-VERSION` prints a version under
+        # en_US.UTF-8 and "FATAL: main argv conversion failed!" under C, the locale
+        # really is the whole story. If it fails under both, it is not the locale and
+        # the next thing to try is DEBUG=electron-builder on the build step to see the
+        # argv makensis is actually handed. Prints nothing useful on the very first
+        # run of a fresh runner, where the NSIS bundle has not been downloaded yet.
+        continue-on-error: true
+        run: |
+          locale || true
+          mk=$(find "$HOME/.cache/electron-builder" /var/lib/gitea-runner/.cache/electron-builder \
+                 -type f -name makensis -path '*/linux/*' 2>/dev/null | head -n1)
+          echo "makensis: ${mk:-<not downloaded yet>}"
+          [ -n "$mk" ] || exit 0
+          echo "--- LC_ALL=en_US.UTF-8"
+          LC_ALL=en_US.UTF-8 LANG=en_US.UTF-8 "$mk" -VERSION || echo "failed, exit $?"
+          echo "--- LC_ALL=C"
+          LC_ALL=C LANG=C "$mk" -VERSION || echo "failed, exit $?"
+
       - name: Build Windows installers + Linux AppImage
         shell: pwsh
         env: