fix(documents): remove bottom panel localStorage persistence

The panel was restoring its open/tab/height state from localStorage,
causing the discussion drawer to reopen on every subsequent page visit
even without a ?commentId= param. Removed all LS_KEY_* constants, the
savedOpen/savedTab/savedHeight restore logic, and the persistence
$effect. The panel now always starts closed (or opens to metadata when
the document has no file yet), and the discussion tab opens exclusively
via the commentId deep-link query param.

Also add .svelte-kit-backup/ to .gitignore and .prettierignore to
prevent lint failures from the root-owned Docker-generated directory.

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

frontend/.prettierignore

@@ -9,6 +9,7 @@ bun.lockb
 /static/
 
 # Generated files
+/.svelte-kit-backup/
 /src/lib/generated/
 /src/lib/paraglide/
 /src/lib/paraglide_bak*/