Pick the text stage's font with a font chooser

A new Font param type (ParamSpec::makeFont) is a font family as the
machine names it. The interface shows it as a family-only
FontDialogButton; the command line and the project file treat it as a
plain string, as they did the Text param it replaces, so existing
projects and --font-family keep working. A family the machine lacks is
kept as written. Bumps the version to 0.8.1.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/core/params.cpp

@@ -100,6 +100,17 @@ ParamSpec ParamSpec::makeColor(std::string key, std::string label, std::string d
     return spec;
 }
 
+ParamSpec ParamSpec::makeFont(std::string key, std::string label, std::string defaultFamily,
+                              std::string help) {
+    ParamSpec spec;
+    spec.type = Type::Font;
+    spec.key = std::move(key);
+    spec.label = std::move(label);
+    spec.help = std::move(help);
+    spec.defaultChoice = std::move(defaultFamily);
+    return spec;
+}
+
 namespace {
 
 const ParamValue* find(const ParamValues& values, const std::string& key) {
@@ -121,6 +132,7 @@ ParamValue paramDefault(const ParamSpec& spec) {
         case ParamSpec::Type::Path:
         case ParamSpec::Type::Text:
         case ParamSpec::Type::Color:
+        case ParamSpec::Type::Font:
             break;
     }
     return spec.defaultChoice;