fix: polish reading display controls

[mattenarle10]

summary

• fix selected editor text replacement by keeping selection sync one-way from preview to editor
• add reading size, reading width, and prose font controls to the existing display menu
• add quick reading zoom controls in the reading-mode titlebar
• update the Gemini theme to a white/gray/blue palette
• keep hidden toolbar collapsed with a compact macOS traffic-light-safe row

issues

Fixes #83
Fixes #84
Fixes #85
Fixes #86
Fixes #87
Fixes #88

tests

• bun test
• bun run build
• cargo check --manifest-path src-tauri/Cargo.toml

notes

• no release/tag in this PR
• system font applies to preview/reading prose; the editor stays monospaced