Add monitor-only mode: suppress all actions without stopping EMS

New internal/monitor package provides a thread-safe, file-backed toggle.
Flag file presence (/etc/ems/monitor-only) = monitor-only active, survives
reboots. Deletion = resume normal operation.

Behaviour when active:
- EMS continues polling Prometheus, running the decision engine, and
  updating the status page every 2 minutes — full visibility maintained
- All actuator calls suppressed (Shelly switches + Viessmann WW writes)
- Suppressed actions logged as [MONITOR-ONLY] for audit trail
- Startup warns if flag file is already present

Web UI:
- Amber banner "⏸ Monitor-Only — Keine Schaltvorgänge" with inline
  "▶ Automatik" resume button when active
- Small unobtrusive "⏸ Monitor-Only" button at page bottom when inactive
- POST /monitor endpoint toggles state and redirects back to status page

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

internal/monitor/mode.go

@@ -0,0 +1,61 @@
+// Package monitor provides a persistent monitor-only mode toggle.
+// When active, the EMS continues polling and showing the status page
+// but suppresses all actuator calls (Shelly switches, Viessmann API).
+// State is stored as a flag file: presence = active, absence = normal.
+package monitor
+
+import (
+	"os"
+	"sync"
+)
+
+// Mode is a thread-safe, file-backed monitor-only flag.
+type Mode struct {
+	mu       sync.RWMutex
+	active   bool
+	filePath string
+}
+
+// New creates a Mode and loads the current state from the flag file.
+// If filePath is empty, the mode is in-memory only (not persistent).
+func New(filePath string) *Mode {
+	m := &Mode{filePath: filePath}
+	if filePath != "" {
+		_, err := os.Stat(filePath)
+		m.active = err == nil // file exists → monitor-only
+	}
+	return m
+}
+
+// IsActive reports whether monitor-only mode is currently enabled.
+func (m *Mode) IsActive() bool {
+	m.mu.RLock()
+	defer m.mu.RUnlock()
+	return m.active
+}
+
+// Set enables or disables monitor-only mode and persists the change to disk.
+func (m *Mode) Set(active bool) error {
+	m.mu.Lock()
+	defer m.mu.Unlock()
+
+	m.active = active
+
+	if m.filePath == "" {
+		return nil // in-memory only
+	}
+
+	if active {
+		f, err := os.Create(m.filePath)
+		if err != nil {
+			return err
+		}
+		return f.Close()
+	}
+
+	err := os.Remove(m.filePath)
+	if os.IsNotExist(err) {
+		return nil
+	}
+	return err
+}