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.lock

@@ -4704,9 +4704,10 @@ checksum = "3e2e65a1a2e43cfcb47a895c4c8b10d1f4a61097f9f254f183aee60cad9c651d"
 
 [[package]]
 name = "margin-app"
-version = "0.1.3"
+version = "0.1.4"
 dependencies = [
  "base64 0.22.1",
+ "fontdb",
  "harper-core",
  "serde",
  "serde_json",