feat(photo-editor)!: add explicit header schemes and prefixed selectors

[rdlabo]

Summary

• require consumers to choose headerButtonColorScheme: 'light' | 'dark' for both PhotoEditorPage and PhotoViewerPage
• apply ambient-independent Ionic button colors and ship a scoped optional iOS 26 adapter through the public css/* export
• rename public component selectors to the package-prefixed rdlabo-photo-editor and rdlabo-photo-viewer
• replace the framework-focused migration document with a library-specific v21 to v22 guide linked from the root and package READMEs
• declare the Ionic 9 peer dependency used at runtime by scroll-header
• expand meaningful unit coverage across kit, photo-editor, scroll-header, and scroll-strategies
• fix browser photo selection so invalid, empty, aborted, read-failure, and resize-failure paths settle cleanly and permit reselection
• fix smooth virtual scrolling to update only the viewport orientation's axis
• cover both header schemes with unit tests and real hydrated Ionic modal E2E tests

Breaking changes

• headerButtonColorScheme is required in PhotoEditorProps and PhotoViewerProps; consumers select the scheme matching the final ion-toolbar appearance.
• PhotoViewerProps.imageUrls is now correctly typed as required, matching the existing required component input.
• Direct-template selectors change as follows:

Before	After
<app-editor-image>	<rdlabo-photo-editor>
<app-photo-image>	<rdlabo-photo-viewer>

ModalController consumers that present the exported component classes do not need a selector-related change. See docs/migration.md for the complete consumer migration.

Verification

• npm run lint
• npm run test (1,048/1,048: demo 11, photo-editor 33, scroll-header 18, scroll-strategies 42, kit 944)
• npm run prebuild
• npm run build
• npx playwright test e2e/photo-editor-ios26.spec.ts (3/3)
• npm pack --dry-run ./dist/photo-editor
• public CSS subpath verified with import.meta.resolve
• git diff --check; no focused or skipped specs
• manager review: APPROVE
• OSS maintainer review: APPROVE