feat(webui): sync host clock and timezone from browser

Adds POST /api/system/time-sync so the dashboard's "Sync with this
browser" button can set the host's date, time, and IANA timezone from
the client's browser — useful for appliances with no NTP/network
access to keep their own clock straight. Disables NTP before writing
the manual time so it doesn't get immediately overwritten.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

audit/internal/webui/api.go

@@ -1483,6 +1483,61 @@ func (h *handler) handleAPISystemShutdown(w http.ResponseWriter, r *http.Request
 	writeJSON(w, map[string]string{"status": "shutting down"})
 }
 
+// timezoneNameRE matches IANA timezone identifiers like "Europe/Moscow" or "UTC".
+var timezoneNameRE = regexp.MustCompile(`^[A-Za-z0-9_+\-]+(/[A-Za-z0-9_+\-]+)*$`)
+
+func validTimezoneName(tz string) bool {
+	if tz == "" || !timezoneNameRE.MatchString(tz) {
+		return false
+	}
+	_, err := os.Stat(filepath.Join("/usr/share/zoneinfo", tz))
+	return err == nil
+}
+
+// handleAPISystemTimeSync sets the host's timezone and wall-clock time from
+// values supplied by the client's browser (used when the appliance has no
+// network/NTP access to keep its own clock in sync).
+func (h *handler) handleAPISystemTimeSync(w http.ResponseWriter, r *http.Request) {
+	var req struct {
+		Timezone string `json:"timezone"`
+		EpochMS  int64  `json:"epoch_ms"`
+	}
+	if err := json.NewDecoder(r.Body).Decode(&req); err != nil {
+		writeError(w, http.StatusBadRequest, "invalid request body")
+		return
+	}
+	if req.EpochMS <= 0 {
+		writeError(w, http.StatusBadRequest, "epoch_ms required")
+		return
+	}
+
+	var out strings.Builder
+
+	if req.Timezone != "" {
+		if !validTimezoneName(req.Timezone) {
+			writeError(w, http.StatusBadRequest, "invalid timezone")
+			return
+		}
+		if b, err := exec.Command("timedatectl", "set-timezone", req.Timezone).CombinedOutput(); err != nil {
+			writeError(w, http.StatusInternalServerError, "set-timezone failed: "+strings.TrimSpace(string(b)))
+			return
+		}
+		fmt.Fprintf(&out, "timezone set to %s\n", req.Timezone)
+	}
+
+	// Manual time only sticks if NTP sync is off.
+	_ = exec.Command("timedatectl", "set-ntp", "false").Run()
+
+	sec := req.EpochMS / 1000
+	if b, err := exec.Command("date", "-u", "-s", "@"+strconv.FormatInt(sec, 10)).CombinedOutput(); err != nil {
+		writeError(w, http.StatusInternalServerError, "set-time failed: "+strings.TrimSpace(string(b)))
+		return
+	}
+	out.WriteString("system clock synced\n")
+
+	writeJSON(w, map[string]string{"status": "ok", "output": out.String()})
+}
+
 // ── Tools ─────────────────────────────────────────────────────────────────────
 
 var standardTools = []string{