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/constants.ts

@@ -8,9 +8,9 @@ export const ALL_ALLOWED_EXTENSIONS = new Set([...MODEL_EXTENSIONS, ...TEXTURE_E
 
 export const REQUIRED_TEXTURES = ['roughness', 'normal', 'metalness', 'color', 'displace'] as const
 
-export const VALID_DESTINATIONS = new Set([
+export const VALID_DESTINATIONS = new Set<string>([
   'farm', 'map', 'powergrid', 'workshop', 'general', 'environment',
-] as const)
+])
 
 export const DESTINATIONS = [
   { value: 'farm', label: 'Farm' },