fix: resolve lint and type-check issues introduced by persons redesign

- Cast PersonSummaryDTO array to concrete type in +page.server.ts (all
  fields are optional in the generated type but always populated at runtime)
- Cast mockLocals/mockLocalsWriter to `any` in persons detail spec to
  match the pre-existing test pattern used throughout the codebase
- Add .svelte-kit-backup/ to .gitignore and .prettierignore to prevent
  lint failures from Docker-owned leftover .svelte-kit directory

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/.prettierignore

@@ -8,6 +8,10 @@ bun.lockb
 # Miscellaneous
 /static/
 
+# Build artifacts
+/.svelte-kit/
+/.svelte-kit-backup/
+
 # Generated files
 /src/lib/generated/
 /src/lib/paraglide/