fix: revert leftover Project→Client mislabels, drop tracked .env.backfill

Rename in 91045a1 was only partially reverted by later commits, leaving
~20 UI spots calling the project entity "Client" while table headers
already said "Project". Reverted all of them plus the /clients/:id
route (now /projects/:id canonical again, nothing was live yet).

Also: stop tracking .env.backfill (had a live admin password/token
committed - rotate those credentials), remove stale pre-redesign
layout.md superseded by REDESIGN-LAYOUT2.md.

.gitignore

@@ -11,6 +11,7 @@ node_modules
 dist
 dist-ssr
 *.local
+.env.backfill
 
 # Editor directories and files
 .vscode/*