feat: Add runtime config, trusted senders, JMAP identities, and UI improvements

- Runtime environment variables for Docker-friendly configuration
- Trusted senders list for automatic image loading
- JMAP identities for proper sender address
- Improved email composer readability
- Horizontal scroll for wide HTML emails

hooks/use-config.ts

@@ -0,0 +1,93 @@
+"use client";
+
+import { useState, useEffect } from 'react';
+
+interface AppConfig {
+  appName: string;
+  jmapServerUrl: string;
+  isLoading: boolean;
+  error: string | null;
+}
+
+// Cache the config to avoid multiple fetches
+let configCache: { appName: string; jmapServerUrl: string } | null = null;
+let configPromise: Promise<{ appName: string; jmapServerUrl: string }> | null = null;
+
+async function fetchConfig(): Promise<{ appName: string; jmapServerUrl: string }> {
+  // Return cached config if available
+  if (configCache) {
+    return configCache;
+  }
+
+  // If a fetch is already in progress, wait for it
+  if (configPromise) {
+    return configPromise;
+  }
+
+  // Start a new fetch
+  configPromise = fetch('/api/config')
+    .then((res) => {
+      if (!res.ok) {
+        throw new Error('Failed to fetch config');
+      }
+      return res.json();
+    })
+    .then((data) => {
+      configCache = data;
+      return data;
+    })
+    .finally(() => {
+      configPromise = null;
+    });
+
+  return configPromise;
+}
+
+/**
+ * Hook to fetch runtime configuration
+ *
+ * Fetches app configuration from /api/config endpoint, which reads
+ * environment variables at runtime (not build time).
+ *
+ * The config is cached after first fetch to avoid unnecessary requests.
+ */
+export function useConfig(): AppConfig {
+  const [config, setConfig] = useState<AppConfig>({
+    appName: configCache?.appName || 'Webmail',
+    jmapServerUrl: configCache?.jmapServerUrl || '',
+    isLoading: !configCache,
+    error: null,
+  });
+
+  useEffect(() => {
+    // If already cached, no need to fetch
+    if (configCache) {
+      setConfig({
+        appName: configCache.appName,
+        jmapServerUrl: configCache.jmapServerUrl,
+        isLoading: false,
+        error: null,
+      });
+      return;
+    }
+
+    fetchConfig()
+      .then((data) => {
+        setConfig({
+          appName: data.appName,
+          jmapServerUrl: data.jmapServerUrl,
+          isLoading: false,
+          error: null,
+        });
+      })
+      .catch((err) => {
+        setConfig((prev) => ({
+          ...prev,
+          isLoading: false,
+          error: err.message,
+        }));
+      });
+  }, []);
+
+  return config;
+}