UI: ask once about anonymous usage; Help › Settings › Privacy (#238)

The first run asks whether to share anonymous usage (only when the host
says this run may), the answer and a random install id are kept in the
preferences, and Help › Settings… changes it. Commands are counted by
where they came from (ui, palette, script; mcp when the MCP server says
so), and cancelled forms and clicks on commands that cannot run yet are
recorded. The preferences go through a PrefsStore (a file, or the
browser's storage). Control channel: ui.settings, ui.prompt answers.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01UhhZMBTviDQxZvwnyP89fo

crates/ui-egui/src/appbar.rs

@@ -340,9 +340,12 @@ fn help_menu(app: &mut ModelApp, ui: &mut egui::Ui) {
         .button("Report a Bug…")
         .on_hover_text("Draft a GitHub issue with the version, the OS and the last error; nothing is sent until you submit it")
         .clicked();
+    let settings = ui.button("Settings…").on_hover_text("Privacy: anonymous usage sharing").clicked();
     ui.label(egui::RichText::new(format!("FieldCraft {}", crate::VERSION)).color(t.text_dim));
     if report {
         app.report_bug(None);
+    } else if settings {
+        app.open_settings();
     }
     if let Some(n) = open {
         app.ui.menu = None;