feat: diagnostic log (tracing)

The app now writes a diagnostic log to nexus.log in the data directory
(0600, rotated at 5 MiB): connection attempts and results, remote
closes, reconnect scheduling, host-key/certificate checks with the
policy applied, and credentials that couldn't be stored (kind only).
Credentials and terminal data are never logged. RUST_LOG sets the level
and also mirrors the log to stderr. The remaining eprintln! diagnostics
in storage and URL handling go through it too.

rust/app/src/widgets.rs

@@ -136,7 +136,7 @@ pub fn is_web_url(url: &str) -> bool {
 /// Opens `url` in the browser if it is a web link; returns false (and opens nothing) otherwise.
 pub fn open_web_url(ctx: &egui::Context, url: &str) -> bool {
     if !is_web_url(url) {
-        eprintln!("refusing to open non-web URL {url:?}");
+        tracing::warn!(url, "refusing to open a non-web URL");
         return false;
     }
     ctx.open_url(egui::OpenUrl::new_tab(url.trim()));