feat(login): dev-only prefill from VITE_ADMIN_EMAIL/PASSWORD

When import.meta.env.DEV is true, the login form's email and password
fields are populated from the corresponding env vars. Shaves the manual
re-typing during dev iteration.

Production builds get empty strings regardless of build-time env values:
the prefill is gated on import.meta.env.DEV (which Vite replaces with
literal `false` at build time, so the surrounding ternary tree-shakes
and the env values can't bleed into the prod bundle even if accidentally
set in the build env).

Files:
- src/vite-env.d.ts (new): ImportMetaEnv augmentation with the four
  VITE_* vars we use (admin email/password, dev directus/processor URLs).
  Gives proper typing under strict mode.
- src/ui/pages/login.tsx: devDefaults computed once at module scope from
  import.meta.env. Form's defaultValues uses it.
- .env.example: documents VITE_ADMIN_EMAIL and VITE_ADMIN_PASSWORD with
  examples; notes the prod-ignore guarantee.
- .gitignore: adds *.env (defensive — complements the existing *.local
  pattern). .env.example stays committable (doesn't end in .env).

.gitignore

@@ -25,3 +25,4 @@ src/routeTree.gen.ts
 *.njsproj
 *.sln
 *.sw?
+*.env