Bundles several in-progress changes from the working tree:
- App appearance / look-and-feel: customizable app theme profiles
(colors, dark interface) with a settings UI and persistence.
- New "Broncode" (source code) slide type: dark code sheet with
syntax highlighting, a dedicated editor with a language picker,
and Marp markdown round-trip via a fenced code block.
- Presenter: eliminate the brief black frame between slides by
precaching neighbouring slide images and enabling gaplessPlayback,
so recordings stay clean.
Adds round-trip tests for the code slide and translations for the
new strings across all supported languages.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
