docs(readme): restructure and improve README readability}
- Reorganize features into categorized sections (core highlights, UI, navigation, settings, other) - Rename "Releases / Downloadable installer" to "Installation" with clearer instructions - Convert flat URL links list into a structured markdown bullet list under "Links" - Consolidate development guidelines into concise bullet points - Rename "Flathub" to "Flatpak" and "MacOS Compatibility" to "macOS" - Improve formatting consistency: add horizontal rules between sections, fix trailing whitespace - Reword verbose descriptions into shorter, more direct language throughout - Clean up screenshot alt text and image reference