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/proofshot-artifacts/2026-03-27_10-49-23/metadata.json

@@ -0,0 +1,6 @@
+{
+	"branch": "feature/68-new-document-file-first",
+	"commitSha": "53b482c5f24688ca3426d434c832e8d24acfee4c",
+	"startedAt": "2026-03-27T10:49:23.106Z",
+	"description": null
+}

frontend/proofshot-artifacts/2026-03-27_10-49-39/metadata.json

@@ -0,0 +1,6 @@
+{
+	"branch": "feature/68-new-document-file-first",
+	"commitSha": "53b482c5f24688ca3426d434c832e8d24acfee4c",
+	"startedAt": "2026-03-27T10:49:39.204Z",
+	"description": null
+}

frontend/proofshot-artifacts/2026-03-27_10-51-02/metadata.json

@@ -0,0 +1,6 @@
+{
+	"branch": "feature/68-new-document-file-first",
+	"commitSha": "53b482c5f24688ca3426d434c832e8d24acfee4c",
+	"startedAt": "2026-03-27T10:51:02.177Z",
+	"description": null
+}