Fix color picker teardown on form rebuild

[sjh9714]

Fixes #3567.

This disposes rebuilt color picker form widgets so Spectrum can remove its generated .sp-container from the document body instead of leaving it behind on each form rebuild.

Changes:

• Add ColorPicker.delete() to destroy the Spectrum instance before removing the widget.
• Add form element disposal in InputForm rebuild/delete paths.
• Dispose an existing dialog form before rebuilding or deleting a dialog.

Validation:

• npm ci
• npx tsc --project tsconfig.json --noEmit
• npm run build-web
• npm run build-electron
• git diff --check