feat(editors): support onKeyboardEvent & enableKeyboardShortcuts in sub-editors (#837)

[hm21]

Fixes #837

Problem

MainEditorCallbacks.onKeyboardEvent and MainEditorConfigs.enableKeyboardShortcuts only affected the main editor. In sub-editors there was no way to intercept or disable keyboard shortcuts — e.g. the R/F keys in the crop-rotate editor always triggered rotate/flip and onKeyboardEvent was never called.

Changes

Shared callback (onKeyboardEvent)

• Moved onKeyboardEvent into the shared StandaloneEditorCallbacks base (plus a handleKeyboardEvent helper). Every standalone editor's callbacks (crop-rotate, paint, filter, tune, blur) now exposes it, and MainEditorCallbacks inherits it (no behavior change for the main editor).

Centralized handling (StandaloneEditorState)

• The mixin now registers a single keyboard handler that:
  1. returns early when enableKeyboardShortcuts is false,
  2. forwards the event to the editor's onKeyboardEvent (consumes it when that returns true),
  3. otherwise delegates to an overridable onCustomKeyEvent hook for built-in shortcuts.

Per-editor wiring

• Crop-rotate & paint: built-in shortcuts moved into onCustomKeyEvent; their own addHandler/removeHandler registration was removed; new enableKeyboardShortcuts config option added (default true).
• Filter, tune, blur: forward keyboard events to their callbacks (they have no built-in shortcuts).

Usage

ProImageEditorConfigs(
  cropRotateEditor: CropRotateEditorConfigs(
    enableKeyboardShortcuts: false, // disable R/F/arrows/zoom shortcuts
  ),
)

ProImageEditorCallbacks(
  cropRotateEditorCallbacks: CropRotateEditorCallbacks(
    onKeyboardEvent: (event) {
      if (event.logicalKey == LogicalKeyboardKey.keyR) return true; // consume
      return false;
    },
  ),
)

Tests

Added a Keyboard shortcuts group to the crop-rotate editor tests: R rotates by default, enableKeyboardShortcuts: false ignores it, and an onKeyboardEvent returning true consumes it (no rotation). Full suite (431 tests) passes and dart analyze lib is clean.