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/client-types.ts

@@ -2,14 +2,14 @@
 // Client-side types — used by components and hooks (no Node.js Buffer)
 // ---------------------------------------------------------------------------
 
-export type FileStatus = 'pending' | 'uploading' | 'success' | 'error'
+type FileStatus = 'pending' | 'uploading' | 'success' | 'error'
 
 export interface TextureFile {
   name: string
   file: File
 }
 
-export type DriveStatus = 'pending' | 'uploading' | 'success' | 'error' | 'skipped'
+type DriveStatus = 'pending' | 'uploading' | 'success' | 'error' | 'skipped'
 
 export interface FolderEntry {
   folderName: string