fix: fail on a nonzero uv installer exit; POSIX quoting for the sg handoff

The Windows installer only checked that uv.exe exists after running the uv
installer, so a failed install that left an older uv.exe behind was accepted;
it now fails on a nonzero exit code.

sg runs its command with /bin/sh, which need not be bash, so the handoff
after installing Docker quotes each argument as POSIX single quotes instead
of with bash's printf %q.

deployment/install.ps1

@@ -64,6 +64,10 @@ function Install-DocsGPT {
             }
             $shell = (Get-Process -Id $PID).Path
             & $shell -NoProfile -ExecutionPolicy Bypass -File $installer
+            if ($LASTEXITCODE -ne 0) {
+                # Without this, a uv.exe left behind by an older install would be accepted below.
+                throw "The uv installer exited with code $LASTEXITCODE."
+            }
         } finally {
             Remove-Item $installer -Force -ErrorAction SilentlyContinue
         }

deployment/install.sh

@@ -50,6 +50,14 @@ main() {
       die "curl or wget is needed to download $1"
     fi
   }
+  # POSIX single-quote encoding: what needs quoting is decided here, not by the shell that runs it.
+  shell_quote() {
+    local arg out=""
+    for arg in "$@"; do
+      out="$out'$(printf '%s' "$arg" | sed "s/'/'\\\\''/g")' "
+    done
+    printf '%s' "$out"
+  }
   sha256_of() {
     if has shasum; then
       shasum -a 256 "$1" | awk '{print $1}'
@@ -182,9 +190,10 @@ main() {
 
   if [ "$docker_group_pending" = 1 ]; then
     if has sg; then
-      # The docker group applies to new logins; sg gives it to this command now.
+      # The docker group applies to new logins; sg gives it to this command now. sg runs the
+      # command with /bin/sh, which need not be bash, so quote for POSIX sh rather than with %q.
       local command
-      command="$(printf '%q ' "$docsgpt" up "$@")"
+      command="$(shell_quote "$docsgpt" up "$@")"
       if have_tty; then
         exec sg docker -c "$command </dev/tty"
       fi