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/WarningBanner.tsx

@@ -1,3 +1,5 @@
+import { WarningIcon } from '@/components/ui/icons'
+
 interface WarningBannerProps {
   warnings: string[]
 }
@@ -8,13 +10,7 @@ export default function WarningBanner({ warnings }: WarningBannerProps) {
   return (
     <div className="mt-2 px-3 py-2 bg-yellow-900/20 border border-yellow-700/30 rounded-lg">
       <div className="flex items-center gap-2 text-xs text-yellow-400">
-        <svg className="w-4 h-4" fill="none" viewBox="0 0 24 24" stroke="currentColor" strokeWidth={2}>
-          <path
-            strokeLinecap="round"
-            strokeLinejoin="round"
-            d="M12 9v2m0 4h.01m-6.938 4h13.856c1.54 0 2.502-1.667 1.732-3L13.732 4c-.77-1.333-2.694-1.333-3.464 0L3.34 16c-.77 1.333.192 3 1.732 3z"
-          />
-        </svg>
+        <WarningIcon className="w-4 h-4" />
         <span>Textures manquantes : {warnings.join(', ')}</span>
       </div>
     </div>