refactor(renderer): split book.ts into 12 focused modules

- Extract chapter-list, editor, exports, full-book, search, toolbar, ui, welcome, comments, types
- Add shared native-doc-core.ts for document contracts
- Update main process, menu, and native-doc for new structure
- Theme and CSS updates

.impeccable/config.json

@@ -0,0 +1,49 @@
+{
+  "detector": {
+    "ignoreRules": [],
+    "ignoreFiles": [
+      "releases/linux-unpacked/LICENSES.chromium.html"
+    ],
+    "ignoreValues": [
+      {
+        "rule": "overused-font",
+        "value": "inter",
+        "createdAt": "2026-08-29T16:09:55.801Z",
+        "reason": "Folio writing app intentionally uses Inter for UI shell and bundles common writing fonts for user choice"
+      },
+      {
+        "rule": "overused-font",
+        "value": "roboto",
+        "createdAt": "2026-08-29T16:10:01.545Z",
+        "reason": "Bundled writing font choice"
+      },
+      {
+        "rule": "overused-font",
+        "value": "open sans",
+        "createdAt": "2026-08-29T16:10:01.593Z",
+        "reason": "Bundled writing font choice"
+      },
+      {
+        "rule": "overused-font",
+        "value": "lato",
+        "createdAt": "2026-08-29T16:10:01.642Z",
+        "reason": "Bundled writing font choice"
+      },
+      {
+        "rule": "overused-font",
+        "value": "montserrat",
+        "createdAt": "2026-08-29T16:10:01.696Z",
+        "reason": "Bundled writing font choice"
+      },
+      {
+        "rule": "clipped-overflow-container",
+        "value": "*",
+        "files": [
+          "src/renderer/index.html"
+        ],
+        "createdAt": "2026-08-29T16:12:00.835Z",
+        "reason": "Hidden chapter views are display:none; main editor fixed to visible for dropdowns"
+      }
+    ]
+  }
+}