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/e2e/.auth/user.json

@@ -5,7 +5,7 @@
       "value": "de",
       "domain": "localhost",
       "path": "/",
-      "expires": 1808896929.897686,
+      "expires": 1809337570.90398,
       "httpOnly": false,
       "secure": false,
       "sameSite": "Lax"
@@ -15,7 +15,7 @@
       "value": "Basic%20YWRtaW46YWRtaW4xMjM%3D",
       "domain": "localhost",
       "path": "/",
-      "expires": 1774423330.233039,
+      "expires": 1774863971.187596,
       "httpOnly": true,
       "secure": false,
       "sameSite": "Strict"