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/services/README.md

@@ -71,7 +71,6 @@ The following table lists ALL services in the system as of 2025-09-01:
 | customerEventsService.ts   | Handles customer event tracking and audit logs            | Analytics  |
 | dialogService.ts           | Provides dialog and modal management                      | UI         |
 | extensionService.ts        | Manages extension registration and lifecycle              | Extensions |
-| keybindingService.ts       | Handles keyboard shortcuts and keybindings                | Input      |
 | litegraphService.ts        | Provides utilities for working with the LiteGraph library | Graph      |
 | load3dService.ts           | Manages 3D model loading and visualization                | 3D         |
 | mediaCacheService.ts       | Manages media file caching with blob storage and cleanup  | Media      |