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/upload/NoChangesModal.tsx

@@ -1,3 +1,6 @@
+import Modal, { ModalActions } from '@/components/ui/Modal'
+import { CheckIcon } from '@/components/ui/icons'
+
 interface NoChangesModalProps {
   destination: string
   folderName: string
@@ -12,52 +15,27 @@ export default function NoChangesModal({
   onModify,
 }: NoChangesModalProps) {
   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="no-changes-title"
-    >
-      <div className="bg-black-900 border border-white/20 rounded-2xl p-6 max-w-md w-full mx-4 space-y-4">
-        <div className="flex items-center gap-3">
-          <div className="w-10 h-10 rounded-full bg-gray-700/50 flex items-center justify-center shrink-0">
-            <svg className="w-5 h-5 text-gray-400" fill="none" viewBox="0 0 24 24" stroke="currentColor" strokeWidth={2}>
-              <path
-                strokeLinecap="round"
-                strokeLinejoin="round"
-                d="M5 13l4 4L19 7"
-              />
-            </svg>
-          </div>
-          <div>
-            <h3 id="no-changes-title" className="text-sm font-semibold text-gray-100">
-              Aucun changement detecte
-            </h3>
-            <p className="text-xs text-gray-400 mt-0.5">
-              Le dossier <span className="font-mono text-gray-300">{destination}/{folderName}</span> est identique au contenu distant. Rien a envoyer.
-            </p>
-          </div>
+    <Modal ariaLabelledBy="no-changes-title">
+      <div className="flex items-center gap-3">
+        <div className="w-10 h-10 rounded-full bg-gray-700/50 flex items-center justify-center shrink-0">
+          <CheckIcon className="w-5 h-5 text-gray-400" />
         </div>
-
-        <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"
-          >
-            Annuler
-          </button>
-          <button
-            onClick={onModify}
-            className="flex-1 bg-white text-[#000000] font-medium text-sm
-                       py-2.5 px-4 rounded-xl transition-colors duration-150
-                       hover:bg-gray-200"
-          >
-            Modifier
-          </button>
+        <div>
+          <h3 id="no-changes-title" className="text-sm font-semibold text-gray-100">
+            Aucun changement detecte
+          </h3>
+          <p className="text-xs text-gray-400 mt-0.5">
+            Le dossier <span className="font-mono text-gray-300">{destination}/{folderName}</span> est identique au contenu distant. Rien a envoyer.
+          </p>
         </div>
       </div>
-    </div>
+
+      <ModalActions
+        cancelLabel="Annuler"
+        confirmLabel="Modifier"
+        onCancel={onCancel}
+        onConfirm={onModify}
+      />
+    </Modal>
   )
 }