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>

tests/affine-cloud/e2e/share-page-2.spec.ts

@@ -117,7 +117,9 @@ test('Should show no permission page when the share page is not found', async ({
   await page.goto('http://localhost:8080/workspace/abc/123');
 
   await expect(
-    page.getByText('You do not have access or this content does not exist.')
+    page.getByText(
+      'Sorry, you do not have access or this content does not exist...'
+    )
   ).toBeVisible();
 });