fix(core): shared page mode syncing (#14756)

### Summary
This fixes a few inconsistencies in shared page behavior:
fixes https://github.com/toeverything/AFFiNE/issues/14751
- shared pages now open in the correct published mode when the URL does
not already include ?mode=...
- switching between page and edgeless in shared mode now keeps the URL
query param in sync
- the default Copy Link action now follows the current editor mode
- shared viewers can toggle between page and edgeless mode in readonly
share pages

---

### What Changed
- updated shared page mode resolution to prefer URL mode, with backend
publish mode as fallback
- added query-param syncing for shared page mode changes
- made the default share link copy use:
  - page link in page mode
  - edgeless link in edgeless mode
- allowed EditorModeSwitch to toggle both ways in shared mode
- extracted shared-mode behavior into small hooks to keep share-page.tsx
cleaner

---

### Demo

https://www.loom.com/share/a287172321fb4fc5b94f7c67a39298a9


<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->
## Summary by CodeRabbit

* **New Features**
* Mode switching between page and edgeless no longer blocked by shared
gating; shared pages initialize and respect the resolved editor mode.
* Shared page URLs stay in sync with editor mode and copy-link actions
include/preserve the selected mode.

* **Tests**
* Added tests for publish-mode resolution, query-string mode handling,
and default share-mode behavior.

* **Bug Fixes**
  * Updated shared-page “not found” UI text to match new messaging.
<!-- end of auto-generated comment: release notes by coderabbit.ai -->

---------

Co-authored-by: DarkSky <25152247+darkskygit@users.noreply.github.com>

packages/frontend/core/src/modules/share-menu/view/share-menu/copy-link-button.tsx

@@ -3,6 +3,7 @@ import {
   getSelectedNodes,
   useSharingUrl,
 } from '@affine/core/components/hooks/affine/use-share-url';
+import { getDefaultShareMode } from '@affine/core/components/hooks/affine/use-share-url.utils';
 import { EditorService } from '@affine/core/modules/editor';
 import { useI18n } from '@affine/i18n';
 import type { DocMode } from '@blocksuite/affine/model';
@@ -46,8 +47,8 @@ export const CopyLinkButton = ({
   }, [onClickCopyLink, currentMode, blockIds, elementIds]);
 
   const onCopyLink = useCallback(() => {
-    onClickCopyLink();
-  }, [onClickCopyLink]);
+    onClickCopyLink(getDefaultShareMode(currentMode));
+  }, [currentMode, onClickCopyLink]);
 
   return (
     <div