feat: add WebDAV file browser with auth improvements

Add a new Files section powered by WebDAV for browsing, uploading,
downloading, renaming, and deleting files and folders.

New features:
- WebDAV file browser with grid/list views and breadcrumb navigation
- File upload (drag-and-drop and button), folder creation, rename, delete
- File preview modals for images and other file types
- WebDAV proxy API route to handle authentication
- Navigation rail entry for Files (auto-hidden when WebDAV is unsupported)

Auth improvements:
- Fix premature redirects on calendar, contacts, and settings pages by
  adding explicit auth check on mount before redirecting to login
- Persist active settings tab in localStorage

Other:
- Expose getAuthHeader() and getServerUrl() on JMAPClient
- Add WebDAV store with connection testing and capability detection
- Add i18n translations for file browser in all 8 locales (de, en, es,
  fr, it, ja, nl, pt)

components/files/new-folder-dialog.tsx

@@ -0,0 +1,58 @@
+"use client";
+
+import { useState } from "react";
+import { useTranslations } from "next-intl";
+import { Button } from "@/components/ui/button";
+import { Input } from "@/components/ui/input";
+
+interface NewFolderDialogProps {
+  onConfirm: (name: string) => Promise<void>;
+  onCancel: () => void;
+}
+
+export function NewFolderDialog({ onConfirm, onCancel }: NewFolderDialogProps) {
+  const t = useTranslations("files");
+  const [name, setName] = useState("");
+  const [isSubmitting, setIsSubmitting] = useState(false);
+
+  const handleSubmit = async (e: React.FormEvent) => {
+    e.preventDefault();
+    const trimmed = name.trim();
+    if (!trimmed) return;
+
+    setIsSubmitting(true);
+    try {
+      await onConfirm(trimmed);
+    } finally {
+      setIsSubmitting(false);
+    }
+  };
+
+  return (
+    <div className="fixed inset-0 z-50 flex items-center justify-center bg-black/50" onClick={onCancel}>
+      <div
+        className="bg-background border border-border rounded-lg shadow-lg p-6 w-full max-w-sm mx-4"
+        onClick={(e) => e.stopPropagation()}
+      >
+        <h2 className="text-lg font-semibold mb-4">{t("new_folder")}</h2>
+        <form onSubmit={handleSubmit}>
+          <Input
+            autoFocus
+            value={name}
+            onChange={(e) => setName(e.target.value)}
+            placeholder={t("new_folder_name")}
+            className="mb-4"
+          />
+          <div className="flex justify-end gap-2">
+            <Button type="button" variant="outline" onClick={onCancel} disabled={isSubmitting}>
+              {t("cancel")}
+            </Button>
+            <Button type="submit" disabled={!name.trim() || isSubmitting}>
+              {t("create")}
+            </Button>
+          </div>
+        </form>
+      </div>
+    </div>
+  );
+}