refactor: migrate keybindings to DDD structure (#8369)

## Summary

Migrate keybindings domain to `src/platform/keybindings/` following DDD
principles.

## Changes

- **What**: Consolidate keybinding-related code (types, store, service,
defaults, reserved keys) into a single domain module with flat structure
- Extracted `KeyComboImpl` and `KeybindingImpl` classes into separate
files
- Updated all consumers to import from new location
- Colocated tests with source files
- Updated stores/README.md and services/README.md to remove migrated
entries

## Review Focus

- Verify all import paths were updated correctly
- Check that the flat structure is appropriate (vs nested core/data/ui
layers)

┆Issue is synchronized with this [Notion
page](https://www.notion.so/PR-8369-refactor-migrate-keybindings-to-DDD-structure-2f66d73d36508120b169dc737075fb45)
by [Unito](https://www.unito.io)

---------

Co-authored-by: Subagent 5 <subagent@example.com>
Co-authored-by: Amp <amp@ampcode.com>

src/scripts/app.ts

@@ -59,7 +59,8 @@ import { useExecutionStore } from '@/stores/executionStore'
 import { useExtensionStore } from '@/stores/extensionStore'
 import { useFirebaseAuthStore } from '@/stores/firebaseAuthStore'
 import { useNodeOutputStore } from '@/stores/imagePreviewStore'
-import { KeyComboImpl, useKeybindingStore } from '@/stores/keybindingStore'
+import { KeyComboImpl } from '@/platform/keybindings/keyCombo'
+import { useKeybindingStore } from '@/platform/keybindings/keybindingStore'
 import { useModelStore } from '@/stores/modelStore'
 import { SYSTEM_NODE_DEFS, useNodeDefStore } from '@/stores/nodeDefStore'
 import { useSubgraphStore } from '@/stores/subgraphStore'