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>

browser_tests/tests/dialog.spec.ts

@@ -1,7 +1,7 @@
 import type { Locator } from '@playwright/test'
 import { expect } from '@playwright/test'
 
-import type { Keybinding } from '../../src/schemas/keyBindingSchema'
+import type { Keybinding } from '../../src/platform/keybindings'
 import { comfyPageFixture as test } from '../fixtures/ComfyPage'
 
 test.beforeEach(async ({ comfyPage }) => {