feat: show snippet ID in editor and add trash and export-all options

[faisalahammad]

Summary

Adds the three improvements requested in #555: the snippet ID is shown in the editor sidebar, snippets in the Trash tab can be exported, and a new Export All button downloads every snippet in one file, including trashed ones.

Closes #555

Changes

Snippet ID in editor sidebar (new SnippetIdInfo.tsx)

Before: the ID was only visible in the page URL.
After: saved snippets show their numeric ID in the sidebar settings, next to a copy button. Hidden for unsaved snippets.
Why: users need a reliable way to reference a snippet ID without reading the URL.

Export from Trash (useApplyBulkAction.ts, RowActions.tsx, ManageSnippetCard.tsx)

Before: the Export row action, bulk action, and card menu item were all hidden in the Trash tab.
After: Export is available in the Trash tab in all three places. Table and card view behave the same.
Why: the existing export builder already writes trashed: true for trashed snippets and the importer ignores that field, so these exports stay import compatible and no PHP change is needed.

Export All (SnippetsTableControls.tsx)

Before: no way to export all snippets at once without selecting rows in every tab.
After: a new button in the list toolbar fetches all snippets through the existing REST endpoint (no pagination, all statuses) and downloads one snippets.code-snippets.json file.
Why: get_items with no query parameters already returns the full set including trashed snippets, so a client-side download reuses existing behavior with no new endpoint.

Exports are read only and do not modify snippet data.

Testing

Export All

1. Open Snippets > All Snippets.
2. Click Export All.
3. The downloaded file contains active, inactive, and trashed snippets under snippets.code-snippets.json.

Export from Trash

1. Trash a snippet.
2. In the Trash tab, export it from its row actions.
3. Select rows and use the Export bulk action for the same result.
4. Both files contain the snippet with "trashed": true and import successfully.

Snippet ID

1. Edit a saved snippet.
2. The sidebar shows its ID with a copy button, matching the id query parameter in the URL.

Automated: added Playwright tests for all three flows in tests/e2e/code-snippets-edit.spec.ts, tests/e2e/code-snippets-list.spec.ts, and tests/e2e/helpers/SnippetsTestHelper.ts helper (trashSnippet). Full lint suite and webpack build pass.