### TL;DR
Refactored `SpecProvider` singleton access pattern and reorganized adapter/extension code structure.
### What changed?
- Changed `SpecProvider.getInstance()` to `SpecProvider._` for cleaner singleton access
- Moved adapter/extension code from `_common` directory to dedicated `adapters` and `extensions` folders
- Consolidated adapter extensions into a single file
- Removed unused dependencies from package.json
- Deleted unnecessary schema files
- Extracted `MobileSpecsPatches` class into the mobile patching code
- Updated all references to use the new `SpecProvider._` accessor
### How to test?
- Verify all specs are properly registered and accessible via `SpecProvider._`
- Test adapter functionality for HTML, Markdown, Notion HTML and plain text
- Check mobile-specific features and patches work correctly
- Ensure preview functionality works in both page and edgeless modes
### Why make this change?
- Improves code organization by properly separating adapters and extensions
- Simplifies singleton access pattern
- Removes unnecessary dependencies and files
- Makes the codebase more maintainable by consolidating related functionality
Close [BS-2647](https://linear.app/affine-design/issue/BS-2647/ios返回手势会误触发focus导致弹起键盘)
### What Changes:
- Using `click` event instead of `pointerDown` event to handle selection logic in page block, because gestures on mobile devices can trigger the `pointerDown` event, causing the cursor to update and unexpectedly bring up the virtual keyboard.
Fix issue [AF-2252](https://linear.app/affine-design/issue/AF-2252).
## What Changed?
- Remove useless `generateAnswer` configuration for inline ask ai
- Refactor the common `updateAIPanelConfig` function
- Use empty string instead of meaningless `placeholder` string
- Remove unnecessary high-order function wrappers
