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/.gitignore

@@ -6,6 +6,7 @@ node_modules
 .netlify
 .wrangler
 /.svelte-kit
+/.svelte-kit-backup
 /build
 
 # OS