feat: add interval-based validation for screenshot_delay

- Validate screenshot_delay < 0.8 × timeout (page.goto timeout)
- Validate screenshot_delay < 0.5 × interval (prevent blocking next check)
- Update UI to dynamically calculate max value based on interval
- Add descriptive error messages showing calculated limits
- Add translation key for max value info display

Addresses CommanderStorm's validation requirements from issue #6471

server/model/monitor.js

@@ -1769,8 +1769,20 @@ class Monitor extends BeanModel {
             // screenshot_delay validation
             if (this.screenshot_delay !== undefined && this.screenshot_delay !== null) {
                 const delay = Number(this.screenshot_delay);
-                if (isNaN(delay) || delay < 0 || delay > 30000) {
-                    throw new Error("Screenshot delay must be between 0 and 30000 milliseconds");
+                if (isNaN(delay) || delay < 0) {
+                    throw new Error("Screenshot delay must be a non-negative number");
+                }
+                
+                // Must not exceed 0.8 * timeout (page.goto timeout is interval * 1000 * 0.8)
+                const maxDelayFromTimeout = this.interval * 1000 * 0.8;
+                if (delay >= maxDelayFromTimeout) {
+                    throw new Error(`Screenshot delay must be less than ${maxDelayFromTimeout}ms (0.8 × interval)`);
+                }
+                
+                // Must not exceed 0.5 * interval to prevent blocking next check
+                const maxDelayFromInterval = this.interval * 1000 * 0.5;
+                if (delay >= maxDelayFromInterval) {
+                    throw new Error(`Screenshot delay must be less than ${maxDelayFromInterval}ms (0.5 × interval)`);
                 }
             }
         }

src/lang/en.json

@@ -1058,6 +1058,7 @@
     "Screenshot Delay ({ms})": "Screenshot Delay ({ms})",
     "ms": "ms",
     "screenshotDelayDescription": "Wait this many milliseconds before taking the screenshot.",
+    "screenshotDelayMaxInfo": "Maximum: {max}ms (0.5 × interval).",
     "screenshotDelayWarning": "Higher values keep the browser open longer, which may increase memory usage with many concurrent monitors.",
     "GrafanaOncallUrl": "Grafana Oncall URL",
     "systemService": "System Service",

src/pages/EditMonitor.vue

@@ -274,11 +274,12 @@
                                         type="number"
                                         class="form-control"
                                         min="0"
-                                        max="30000"
+                                        :max="Math.floor(monitor.interval * 1000 * 0.5)"
                                         step="100"
                                     />
                                     <div class="form-text">
                                         {{ $t("screenshotDelayDescription") }}
+                                        {{ $t("screenshotDelayMaxInfo", { max: Math.floor(monitor.interval * 1000 * 0.5) }) }}
                                     </div>
                                     <div class="form-text text-warning">
                                         {{ $t("screenshotDelayWarning") }}