Implement selection state management in Vue Nodes (#5421)

* let canvas continue to own selection state management

* fix merge error

* refactor: use computed instead of watcher for selectedNodeIds

Replace watcher pattern with computed for better Vue idioms:
- More reactive and efficient
- Automatically recomputes when dependencies change
- Simpler, more declarative code

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

* fix: improve injection error handling for selectedNodeIds

Replace silent fallback with explicit error when SelectedNodeIds
is not provided:
- Fail fast instead of silently using empty Set
- Clear error message for debugging
- Prevents nodes appearing unselected due to missing provider

Addresses DrJKL's concern about injection default behavior.

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

* test: improve mocking patterns using vi.mockObject

Replace manual mock interfaces with vi.mockObject for better type safety:
- Use Vitest's built-in mocking utilities instead of manual interfaces
- Properly configure mock return values
- Remove unnecessary type assertions

Addresses DrJKL's feedback on test mocking patterns.

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

* test: extract repeated nodeData for clarity

Extract common test nodeData object to reduce duplication:
- Move repeated VueNodeData object to describe scope
- Replace 6 instances of identical nodeData declarations
- Maintain different nodeData for specific test cases

Addresses DrJKL's suggestion to extract repeated test data.

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

* add type safety to mocks

---------

Co-authored-by: Claude <noreply@anthropic.com>

src/renderer/core/canvas/injectionKeys.ts

@@ -0,0 +1,8 @@
+import type { InjectionKey, Ref } from 'vue'
+
+/**
+ * Injection key for providing selected node IDs to Vue node components.
+ * Contains a reactive Set of selected node IDs (as strings).
+ */
+export const SelectedNodeIdsKey: InjectionKey<Ref<Set<string>>> =
+  Symbol('selectedNodeIds')