Skip the key step when SSH already works, and read the install path live on both sides

Partnership/partnership_onboard.sh

@@ -315,14 +315,33 @@ if [[ "$AM_MIRROR" == true ]]; then
     echo "  Mirror sets up SSH keys, then notifies Owner to run Phase 2."
     echo ""
 
+    # Tested before attempted, the same guard the owner's phase 1 has had all along.
+    #
+    # The key step is a TERMINAL step by design — ssh_setup.sh runs ssh-copy-id, which prompts for
+    # the owner's root password on a first install. Once the operator has done that in a terminal,
+    # pressing ▶ Onboard ran the whole thing again: another ssh-copy-id, this time from the WebGUI
+    # with no TTY to answer the prompt, which fails and aborts the run at Step 1 — so the button
+    # whose entire job is Step 2 could never reach it.
+    #
+    # Working SSH is the actual precondition, not "have we run the setup script". If it already
+    # works there is nothing to install, whichever route installed it.
+    OWNER_IP_PRE=$(resolve_tailscale_ip "$OWNER" 2>/dev/null || true)
     if [[ "$SKIP_SSH" == true ]]; then
         warn "Skipping SSH setup (--skip-ssh)"
+    elif [[ -n "$OWNER_IP_PRE" ]] && timeout "$SSH_TIMEOUT" ssh -i "$SSH_KEY" \
+            -o ConnectTimeout="$SSH_TIMEOUT" -o BatchMode=yes -o StrictHostKeyChecking=no \
+            root@"$OWNER_IP_PRE" exit 0 2>/dev/null; then
+        echo "SSH to $OWNER already works ✅ — key already installed, skipping setup"
     elif bash "$SCRIPT_DIR/ssh_setup.sh" "${EXTRA_FLAGS[@]}"; then
         echo "SSH key ready ✅"
     else
         error "SSH key setup failed"
+        error "Install the key from a terminal on this host — ssh-copy-id needs $OWNER's password,"
+        error "and a WebGUI button has no way to answer that prompt:"
+        error "    bash $SCRIPTS_ROOT/Partnership/partnership_onboard.sh --phase1-only"
         exit 1
     fi
+    unset OWNER_IP_PRE
 
     # Stop after the key when asked. ssh_setup.sh runs ssh-copy-id, which prompts for the
     # owner's root password on a first install — answerable in a terminal, never from the