feat: expose litegraph internal keybindings (#9459)

## Summary

Migrate hardcoded litegraph canvas keybindings (Ctrl+A/C/V, Delete,
Backspace) into the customizable keybinding system so users can remap
them via Settings > Keybindings.

## Changes

- **What**: Register Ctrl+A (SelectAll), Ctrl+C (CopySelected), Ctrl+V
(PasteFromClipboard), Ctrl+Shift+V (PasteFromClipboardWithConnect),
Delete/Backspace (DeleteSelectedItems) as core keybindings in
`defaults.ts`. Add new `PasteFromClipboardWithConnect` command. Remove
hardcoded handling from litegraph `processKey()`, the `app.ts` Ctrl+C/V
monkey-patch, and the `keybindingService` canvas forwarding logic.

Fixes #1082
Fixes #2015

┆Issue is synchronized with this [Notion
page](https://www.notion.so/PR-9459-feat-expose-litegraph-internal-keybindings-31b6d73d3650819a8499fd96c8a6678f)
by [Unito](https://www.unito.io)

src/scripts/app.ts

@@ -676,20 +676,6 @@ export class ComfyApp {
           e.stopImmediatePropagation()
           return
         }
-
-        // Ctrl+C Copy
-        if (e.key === 'c' && (e.metaKey || e.ctrlKey)) {
-          return
-        }
-
-        // Ctrl+V Paste
-        if (
-          (e.key === 'v' || e.key == 'V') &&
-          (e.metaKey || e.ctrlKey) &&
-          !e.shiftKey
-        ) {
-          return
-        }
       }
 
       // Fall through to Litegraph defaults