[RFC] Flatten 9 single-file component subdirectories in `src/components/`

## Problem

`src/components/` has 37 subdirectories, but 9 contain only 1-2 files. These overly-granular directories add navigation overhead without organizational value:

Candidates for consolidation (directories with 1-2 files):
- `appMode/`
- `boundingbox/`
- `chip/`
- `gradientslider/`
- `imagecrop/`
- `loader/`
- `painter/`
- `tab/`
- `breadcrumb/`

## Proposed Fix

Merge single-file directories into related larger directories:
- `appMode/` → inline into `common/` or parent
- `boundingbox/`, `imagecrop/`, `painter/` → `maskeditor/` or `canvas/`
- `chip/`, `tab/`, `loader/`, `breadcrumb/` → `ui/` or `common/`
- `gradientslider/` → `widget/` or `input/`

## Migration Plan

- Single PR per merge
- Update all import paths

## Testing Strategy

- Import path changes only
- `pnpm typecheck` passes

┆Issue is synchronized with this [Notion page](https://www.notion.so/Issue-11086-RFC-Flatten-9-single-file-component-subdirectories-in-src-components-33e6d73d3650816ba4ccd53b1ad352fc) by [Unito](https://www.unito.io)