style(branding): modernize app logo and icons

Replace outdated Fugue-style logo with a modern minimalist design:
- Blue open book with hiragana あ and Latin A characters
- New SVG source files: lwt-logo.svg (outline), lwt-logo-filled.svg, favicon.svg
- Regenerated all raster icons (favicon.ico, apple-touch-icons, lwt_icon variants)

Author: HugoFara