feat: add download + copy file content to context menus and editor

- Add /api/pages/{id}/download and /api/pages/{id}/file-content endpoints
- Add Download + Copy content to shared context menu (_ctx_menu.html)
- Wire handlers in local-workspace and library context menus
- Add Download + Copy content to editor '...' menu for file pages
- Multi-block selection copy/cut/paste with Ctrl+C/X/V shortcuts
- Align page title with blocks start (CSS pseudo-element offset)

app/templates/base.html

@@ -45,6 +45,15 @@
       .page-title-block {
         padding: 16px 8px;
       }
+      .page-title-block::before {
+        display: none;
+      }
+      .page-title-block .page-icon-emoji {
+        margin-left: 0;
+      }
+      .page-title-block .page-title-input {
+        padding-left: 0;
+      }
       .blocks-container {
         padding: 0 4px;
       }