Dramatically improve temperature proxy installation robustness

Users were abandoning Pulse due to catastrophic temperature monitoring setup failures. This commit addresses the root causes:

**Problem 1: Silent Failures**
- Installations reported "SUCCESS" even when proxy never started
- UI showed green checkmarks with no temperature data
- Zero feedback when things went wrong

**Problem 2: Missing Diagnostics**
- Service failures logged only in journald
- Users saw "Something going on with the proxy" with no actionable guidance
- No way to troubleshoot from error messages

**Problem 3: Standalone Node Issues**
- Proxy daemon logged continuous pvecm errors as warnings
- "ipcc_send_rec" and "Unknown error -1" messages confused users
- These are expected for non-clustered/LXC setups

**Solutions Implemented:**

1. **Health Gate in install.sh (lines 1588-1629)**
   - Verify service is running after installation
   - Check socket exists on host
   - Confirm socket visible inside container via bind mount
   - Fail loudly with specific diagnostics if any check fails

2. **Actionable Error Messages in install-sensor-proxy.sh (lines 822-877)**
   - When service fails to start: dump full systemctl status + 40 lines of logs
   - When socket missing: show permissions, service status, and remediation command
   - Include common issues checklist (missing user, permission errors, lm-sensors, etc.)
   - Direct link to troubleshooting docs

3. **Better Standalone Node Detection in ssh.go (lines 585-595)**
   - Recognize "Unknown error -1" and "Unable to load access control list" as LXC indicators
   - Log at INFO level (not WARN) since this is expected behavior
   - Clarify message: "using localhost for temperature collection"

**Impact:**
- Eliminates "green checkmark but no temps" scenario
- Users get immediate actionable feedback on failures
- Standalone/LXC installations work silently without error spam
- Reduces support burden from #571 (15+ comments of user frustration)

Related to #571

scripts/install-sensor-proxy.sh

@@ -821,8 +821,26 @@ fi
 
 if ! systemctl restart pulse-sensor-proxy.service; then
     print_error "Failed to start pulse-sensor-proxy service"
-    print_error "Check service logs:"
-    journalctl -u pulse-sensor-proxy -n 20 --no-pager
+    print_error ""
+    print_error "═══════════════════════════════════════════════════════"
+    print_error "Service Status:"
+    print_error "═══════════════════════════════════════════════════════"
+    systemctl status pulse-sensor-proxy --no-pager --lines=0 2>&1 || true
+    print_error ""
+    print_error "═══════════════════════════════════════════════════════"
+    print_error "Recent Logs (last 40 lines):"
+    print_error "═══════════════════════════════════════════════════════"
+    journalctl -u pulse-sensor-proxy -n 40 --no-pager 2>&1 || true
+    print_error ""
+    print_error "═══════════════════════════════════════════════════════"
+    print_error "Common Issues:"
+    print_error "═══════════════════════════════════════════════════════"
+    print_error "1. Missing user: Run 'useradd --system --no-create-home --group pulse-sensor-proxy'"
+    print_error "2. Permission errors: Check ownership of /var/lib/pulse-sensor-proxy"
+    print_error "3. lm-sensors not installed: Run 'apt-get install lm-sensors && sensors-detect --auto'"
+    print_error "4. Standalone node detection: If you see 'pvecm' errors, this is expected for non-clustered hosts"
+    print_error ""
+    print_error "For more help: https://github.com/rcourtman/Pulse/blob/main/docs/TROUBLESHOOTING.md"
     exit 1
 fi
 
@@ -836,8 +854,26 @@ for i in {1..10}; do
 done
 
 if [[ ! -S "$SOCKET_PATH" ]]; then
-    print_error "Socket did not appear after 10 seconds"
-    print_info "Check service status: systemctl status pulse-sensor-proxy"
+    print_error "Socket did not appear at $SOCKET_PATH after 10 seconds"
+    print_error ""
+    print_error "═══════════════════════════════════════════════════════"
+    print_error "Diagnostics:"
+    print_error "═══════════════════════════════════════════════════════"
+    print_error "Service Status:"
+    systemctl status pulse-sensor-proxy --no-pager 2>&1 || true
+    print_error ""
+    print_error "Socket Directory Permissions:"
+    ls -la /run/pulse-sensor-proxy/ 2>&1 || echo "Directory does not exist"
+    print_error ""
+    print_error "Recent Logs:"
+    journalctl -u pulse-sensor-proxy -n 20 --no-pager 2>&1 || true
+    print_error ""
+    print_error "Common Causes:"
+    print_error "  • Service failed to start (check logs above)"
+    print_error "  • RuntimeDirectory permissions issue"
+    print_error "  • Systemd socket creation failed"
+    print_error ""
+    print_error "Try: systemctl restart pulse-sensor-proxy && watch -n 0.5 'ls -la /run/pulse-sensor-proxy/'"
     exit 1
 fi