refactor(person-mention): hoist LoadState + HoverData into shared types module

Markus flagged the LoadState export from PersonHoverCard.svelte as a
view-vs-orchestrator boundary smell — both files own the same shape, and a
third caller (admin previews, briefwechsel cards) would create a circular
import. Move the types into src/lib/types/personHoverCard.ts so the contract
is module-stable.

Also harden .prettierignore + eslint.config.js so a stray .svelte-kit.old/
backup directory (rotated by SvelteKit during dev) doesn't break the lint
hook — matches the existing .svelte-kit-backup/ convention.

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

frontend/.prettierignore

@@ -11,6 +11,7 @@ bun.lockb
 # Build artifacts
 /.svelte-kit/
 /.svelte-kit-backup/
+/.svelte-kit.old/
 
 # Generated files
 /.svelte-kit-backup/