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/validate-folder.ts

@@ -13,22 +13,15 @@ function getTextureType(filename: string): string | null {
   return null
 }
 
-export function validateFolder(files: File[]): {
-  model?: File
-  textures: TextureFile[]
-  errors: string[]
-  warnings: string[]
-} {
-  const result: {
-    model?: File
-    textures: TextureFile[]
-    errors: string[]
-    warnings: string[]
-  } = {
-    textures: [],
-    errors: [],
-    warnings: [],
-  }
+/** Discriminated union: either valid (with model) or invalid (with errors). */
+export type ValidationResult =
+  | { ok: true; model: File; textures: TextureFile[]; warnings: string[] }
+  | { ok: false; errors: string[] }
+
+export function validateFolder(files: File[]): ValidationResult {
+  const textures: TextureFile[] = []
+  const warnings: string[] = []
+  const errors: string[] = []
 
   const modelFiles = files.filter((f) => {
     const name = f.name.toLowerCase()
@@ -36,9 +29,7 @@ export function validateFolder(files: File[]): {
   })
 
   if (modelFiles.length === 0) {
-    result.errors.push('model.glb ou model.gltf manquant (obligatoire)')
-  } else {
-    result.model = modelFiles[0]
+    return { ok: false, errors: ['model.glb ou model.gltf manquant (obligatoire)'] }
   }
 
   const textureFiles = files.filter((f) => {
@@ -47,17 +38,21 @@ export function validateFolder(files: File[]): {
   })
 
   for (const tf of textureFiles) {
-    result.textures.push({ name: tf.name, file: tf })
+    textures.push({ name: tf.name, file: tf })
   }
 
   const foundTextures = new Set(
-    result.textures.map((t) => t.name.toLowerCase().replace(/\.[^.]+$/, '')),
+    textures.map((t) => t.name.toLowerCase().replace(/\.[^.]+$/, '')),
   )
   for (const req of REQUIRED_TEXTURES) {
     if (!foundTextures.has(req)) {
-      result.warnings.push(`${req}.webp/png/jpg manquant`)
+      warnings.push(`${req}.webp/png/jpg manquant`)
     }
   }
 
-  return result
+  if (errors.length > 0) {
+    return { ok: false, errors }
+  }
+
+  return { ok: true, model: modelFiles[0], textures, warnings }
 }