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

app/api/upload/drive/route.ts

@@ -2,7 +2,6 @@ import { NextRequest, NextResponse } from 'next/server'
 import { validateUploadSecret } from '@/lib/auth'
 import { parseMultiUpload } from '@/lib/parse-upload'
 import {
-  folderExists,
   mkdirRecursive,
   moveFolder,
   uploadFile,
@@ -22,11 +21,11 @@ export const dynamic = 'force-dynamic'
 //   - action: "new" | "replace"
 //
 // Versioning logic:
-//   VF/{folderName}  ← latest version
-//   V1/{folderName}  ← first archive, V2/ second, etc.
+//   VF/{folderName}  <- latest version
+//   V1/{folderName}  <- first archive, V2/ second, etc.
 //
-//   action="new"     → just mkdir + upload into VF/
-//   action="replace" → archive VF → Vx, then re-upload all files into VF/
+//   action="new"     -> just mkdir + upload into VF/
+//   action="replace" -> archive VF -> Vx, then re-upload all files into VF/
 // ---------------------------------------------------------------------------
 
 export async function POST(req: NextRequest) {
@@ -42,11 +41,7 @@ export async function POST(req: NextRequest) {
     )
   }
 
-  // --- Parse files ---
-  // Clone the request before parseMultiUpload consumes the body stream,
-  // so we can read the `action` field separately.
-  const cloned = req.clone()
-
+  // --- Parse files (includes extra fields like "action") ---
   let folderName: string
   let parsedFiles: Awaited<ReturnType<typeof parseMultiUpload>>['files']
   let action: string
@@ -55,10 +50,7 @@ export async function POST(req: NextRequest) {
     const parsed = await parseMultiUpload(req)
     folderName = parsed.folderName
     parsedFiles = parsed.files
-
-    // Read action from the cloned request (parseMultiUpload doesn't expose it)
-    const formData = await cloned.formData()
-    action = (formData.get('action') as string | null)?.trim() || 'new'
+    action = parsed.extra.action?.trim() || 'new'
   } catch (err) {
     const message = err instanceof Error ? err.message : 'Erreur inconnue'
     return NextResponse.json({ success: false, error: message }, { status: 400 })
@@ -75,7 +67,7 @@ export async function POST(req: NextRequest) {
       // 2. Ensure Vx/ exists
       await mkdirRecursive(`${basePath}/${nextVersion}`)
 
-      // 3. Move VF/{folderName} → Vx/{folderName}
+      // 3. Move VF/{folderName} -> Vx/{folderName}
       await moveFolder(vfFolderPath, `${basePath}/${nextVersion}/${folderName}`)
 
       // 4. Re-create VF/{folderName}