feat: initial MQTT v5 debugging tool

- Tauri v2 + Vue 3 + TypeScript + Element Plus
- Connection management (CRUD + enable/disable/test)
- Preset system with {{variable}} placeholders
- Dashboard with live message scrolling + topic tree
- Payload inspector (JSON/Hex/Base64/Plain)
- Protocol-level logging (file + UI dual channel)
- Global variable manager with localStorage persistence
- MQTT v5 property support (user properties, content type, etc.)
- SubAck result tracking with subscribe_tracked API

src/utils/envVarResolver.ts

@@ -0,0 +1,58 @@
+/**
+ * Extract all {{variable}} placeholder names from a template string.
+ * Returns unique variable names in order of first appearance.
+ */
+export function extractVariables(template: string): string[] {
+  const regex = /\{\{(\w+)\}\}/g
+  const names: string[] = []
+  const seen = new Set<string>()
+  let match: RegExpExecArray | null
+  while ((match = regex.exec(template)) !== null) {
+    if (!seen.has(match[1])) {
+      seen.add(match[1])
+      names.push(match[1])
+    }
+  }
+  return names
+}
+
+/**
+ * Replace {{variable}} placeholders with values from bindings.
+ * Unresolved placeholders are left as-is.
+ */
+export function resolveTemplate(
+  template: string,
+  bindings: Record<string, string>,
+): string {
+  return template.replace(/\{\{(\w+)\}\}/g, (_, name: string) => {
+    return bindings[name] ?? `{{${name}}}`
+  })
+}
+
+/**
+ * Extract user property placeholder names from a list of key-value pairs.
+ * Both key and value can contain {{variables}}.
+ */
+export function extractUserPropertyVariables(
+  userProperties: { key: string; value: string }[],
+): string[] {
+  const names = new Set<string>()
+  for (const prop of userProperties) {
+    for (const p of extractVariables(prop.key)) names.add(p)
+    for (const p of extractVariables(prop.value)) names.add(p)
+  }
+  return [...names]
+}
+
+/**
+ * Resolve user property key-value pairs with variable bindings.
+ */
+export function resolveUserProperties(
+  userProperties: { key: string; value: string }[],
+  bindings: Record<string, string>,
+): { key: string; value: string }[] {
+  return userProperties.map((p) => ({
+    key: resolveTemplate(p.key, bindings),
+    value: resolveTemplate(p.value, bindings),
+  }))
+}