feat(fonts): add font picker to book setup

Add a Typography section to book setup with one-click preset pairings,
an advanced body/heading picker, and a live sample. Enumerate installed
fonts via a new list_system_fonts Tauri command (fontdb) so they appear
alongside the bundled set, with a note that system fonts may not embed
in exports.

src-tauri/Cargo.toml

@@ -27,6 +27,7 @@ serde = { version = "1", features = ["derive"] }
 serde_json = "1"
 base64 = "0.22"
 zip = "2"
+fontdb = "0.23"
 typst = "0.14.2"
 typst-pdf = "0.14.2"
 typst-as-lib = "0.15.5"