revert: always rebuild frontend, override caching with _NEED_FRONTEND_BUILD=true (#4427)

* revert: remove frontend build caching from setup scripts

The mtime-based caching introduced in #4404/#4413 can incorrectly skip
frontend builds -- e.g. after git pull when filesystem timestamps are
not preserved, or after our Tailwind v4 discovery that the site-packages
.gitignore must be hidden before vite build (which the cached path
doesn't handle).

Always rebuild the frontend on setup. The build takes ~15s and is
safer than risking a stale dist/.

* revert: disable frontend build caching, keep code commented out

Caching disabled by always setting _NEED_FRONTEND_BUILD=true.
The mtime-based logic is preserved in comments for future re-enabling.

Reasons for disabling:
- Git does not preserve file timestamps, so cached dist/ can appear
  newer than freshly checked-out source after a pull
- Tailwind v4 requires hiding site-packages/.gitignore before vite
  build; the cache path bypasses this, producing broken CSS

* revert: always rebuild frontend, remove mtime caching

* revert: always rebuild frontend, override caching with _NEED_FRONTEND_BUILD=true

studio/setup.ps1

@@ -855,6 +855,7 @@ if ($IsPipInstall) {
         Write-Host "[INFO] Frontend source changed since last build -- rebuilding..." -ForegroundColor Yellow
     }
 }
+$NeedFrontendBuild = $true
 if ($NeedFrontendBuild -and -not $IsPipInstall) {
     Write-Host ""
     Write-Host "Building frontend..." -ForegroundColor Cyan

studio/setup.sh

@@ -59,6 +59,7 @@ if [ -d "$SCRIPT_DIR/frontend/dist" ]; then
         _NEED_FRONTEND_BUILD=false
     fi
 fi
+_NEED_FRONTEND_BUILD=true
 if [ "$_NEED_FRONTEND_BUILD" = false ]; then
     echo "✅ Frontend already built and up to date -- skipping Node/npm check."
 else