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

lib/format-bytes.ts

@@ -3,7 +3,7 @@
 // ---------------------------------------------------------------------------
 
 export function formatBytes(bytes: number): string {
-  if (bytes === 0) return '0 B'
+  if (bytes <= 0) return '0 B'
   const k = 1024
   const sizes = ['B', 'KB', 'MB', 'GB']
   const i = Math.floor(Math.log(bytes) / Math.log(k))