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/config/config.go

@@ -159,6 +159,7 @@ type EMSConfig struct {
 	RecoveryTimeout    string  `yaml:"recovery_timeout"`
 	OverrideTimeout    string  `yaml:"override_timeout"`
 	OverrideMaxImportW float64 `yaml:"override_max_import_w"` // cancel override if grid import exceeds this (0 = disabled)
+	MonitorOnlyFile    string  `yaml:"monitor_only_file"`     // flag file path: presence = monitor-only mode active
 }
 
 func (e *EMSConfig) PollIntervalParsed() time.Duration {