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/git/route.ts

@@ -7,8 +7,8 @@ import { parseMultiUpload } from '@/lib/parse-upload'
 import { compressWithBlender } from '@/lib/blender'
 import { getRemoteFolder, pushAllToGitHub } from '@/lib/github'
 import { buildCommitMessage } from '@/lib/commit-message'
-import { TMP_DIR, MODEL_EXTENSIONS } from '@/lib/constants'
-import type { FileChange } from '@/lib/types'
+import { classifyFileChanges } from '@/lib/diff-files'
+import { TMP_DIR } from '@/lib/constants'
 
 export const runtime = 'nodejs'
 export const dynamic = 'force-dynamic'
@@ -85,8 +85,6 @@ export async function POST(req: NextRequest) {
   }
 
   // --- Detect existing files and classify changes ---
-  // Models: always re-push (compression changes size, can't compare with LFS remote)
-  // Textures: compare by size (not compressed, reliable)
   const folderPath = `public/models/${destination}/${folderName}`
   let remoteFileMap: Map<string, number>
 
@@ -99,48 +97,8 @@ export async function POST(req: NextRequest) {
 
   const isReplace = remoteFileMap.size > 0
 
-  const fileChanges = new Map<string, FileChange>()
-  const changedFilesToPush: { path: string; contentBase64: string }[] = []
-
-  for (const f of filesToPush) {
-    const filename = f.path.split('/').pop() ?? ''
-    const ext = filename.slice(filename.lastIndexOf('.')).toLowerCase()
-    const isModel = MODEL_EXTENSIONS.has(ext)
-
-    if (isModel) {
-      // Model: always re-push since compression makes size comparison unreliable.
-      // Mark as 'unchanged' for the commit message when the folder already exists,
-      // because we can't know if the model really changed after Blender compression.
-      const remoteSize = remoteFileMap.get(filename.toLowerCase())
-      fileChanges.set(filename.toLowerCase(), remoteSize === undefined ? 'new' : 'unchanged')
-      changedFilesToPush.push(f)
-    } else {
-      // Texture: compare by size
-      const localSize = Buffer.from(f.contentBase64, 'base64').length
-      const remoteSize = remoteFileMap.get(filename.toLowerCase())
-
-      if (remoteSize === undefined) {
-        fileChanges.set(filename.toLowerCase(), 'new')
-        changedFilesToPush.push(f)
-      } else if (remoteSize !== localSize) {
-        fileChanges.set(filename.toLowerCase(), 'changed')
-        changedFilesToPush.push(f)
-      } else {
-        fileChanges.set(filename.toLowerCase(), 'unchanged')
-      }
-    }
-  }
-
-  // Files on remote not in the new upload → deleted (orphans)
-  const newFileNames = new Set(filesToPush.map((f) => (f.path.split('/').pop() ?? '').toLowerCase()))
-  const deletedFileNames: string[] = []
-  const deletePaths: string[] = []
-  for (const [name] of remoteFileMap) {
-    if (!newFileNames.has(name)) {
-      deletedFileNames.push(name)
-      deletePaths.push(`${folderPath}/${name}`)
-    }
-  }
+  const { fileChanges, changedFilesToPush, deletedFileNames, deletePaths } =
+    classifyFileChanges(filesToPush, remoteFileMap, folderPath)
 
   // If nothing changed, don't create an empty commit
   if (changedFilesToPush.length === 0 && deletePaths.length === 0) {