refactor: full codebase audit — extract modules, fix type safety, clean dead code

- Extract API helpers from UploadZone into lib/upload-api.ts (FormData builder, checkFolderDiffs, uploadDrive, uploadGit)
- Extract upload orchestration into hooks/useUploadOrchestrator.ts (UploadZone: 489 → 162 lines)
- Extract file diff classification into lib/diff-files.ts (from git route)
- Extract shared SVG icons into components/ui/icons.tsx (7 icons, 0 duplication)
- Extract shared modal wrapper into components/ui/Modal.tsx + ModalActions
- Extract DriveStatusLine sub-component from FolderCard
- Fix checkFolderDiffs silently swallowing auth/network errors (now throws)
- Fix type safety: remove as never casts, add isHttpError type guard, use discriminated union for validateFolder
- Fix nextcloud: cache getConfig, add max bound to findNextVersion, optimize mkdirRecursive (skip PROPFIND)
- Fix drive route: remove req.clone(), extend parseMultiUpload to return extra fields
- Fix commit message: model shown as unchanged with ↔️ on updates (not falsely marked as modified)
- Clean dead code: unused folderExists import, FileStatus/DriveStatus exports, ParsedFile.textureName, getConfig basePath
- Add security headers in next.config.ts (HSTS, X-Content-Type-Options, X-Frame-Options, etc.)
- Update README with new project structure

components/ui/Modal.tsx

@@ -0,0 +1,65 @@
+// ---------------------------------------------------------------------------
+// Shared modal wrapper — handles overlay, centering, dialog role, aria
+// ---------------------------------------------------------------------------
+
+import type { ReactNode } from 'react'
+
+interface ModalProps {
+  ariaLabelledBy: string
+  children: ReactNode
+}
+
+export default function Modal({ ariaLabelledBy, children }: ModalProps) {
+  return (
+    <div
+      className="fixed inset-0 z-50 flex items-center justify-center bg-black/60 backdrop-blur-sm"
+      role="dialog"
+      aria-modal="true"
+      aria-labelledby={ariaLabelledBy}
+    >
+      <div className="bg-black-900 border border-white/20 rounded-2xl p-6 max-w-md w-full mx-4 space-y-4">
+        {children}
+      </div>
+    </div>
+  )
+}
+
+// ---------------------------------------------------------------------------
+// Shared modal footer with two buttons
+// ---------------------------------------------------------------------------
+
+interface ModalActionsProps {
+  cancelLabel: string
+  confirmLabel: string
+  onCancel: () => void
+  onConfirm: () => void
+  /** Tailwind classes for the confirm button (default: white bg) */
+  confirmClassName?: string
+}
+
+export function ModalActions({
+  cancelLabel,
+  confirmLabel,
+  onCancel,
+  onConfirm,
+  confirmClassName = 'bg-white text-[#000000] hover:bg-gray-200',
+}: ModalActionsProps) {
+  return (
+    <div className="flex gap-3">
+      <button
+        onClick={onCancel}
+        className="flex-1 bg-black-700 text-gray-300 font-medium text-sm
+                   py-2.5 px-4 rounded-xl border border-white/10 transition-colors duration-150
+                   hover:bg-black-600"
+      >
+        {cancelLabel}
+      </button>
+      <button
+        onClick={onConfirm}
+        className={`flex-1 font-medium text-sm py-2.5 px-4 rounded-xl transition-colors duration-150 ${confirmClassName}`}
+      >
+        {confirmLabel}
+      </button>
+    </div>
+  )
+}