fix(ui): explain the io errors that build their own message

`explain_io` exists because "Permission denied (os error 13)" answers a
developer's question and not the reader's — its own doc says so. Every
failure routed through `HostOps::notify_err` gets it. Four that build
their notification string by hand did not, and printed the raw error.

The clearest symptom was inside one file: saving a file in the editor
went through `notify_err` and explained itself, while opening the same
file, denied for the same reason, said `os error 13`. Both file-link
openers are changed together — the file tree's opener deliberately shares
its wording with the terminal's (#542), so explaining one and not the
other would have split a pairing that was on purpose.

The `log::warn!` next to each call keeps the exact error. A developer
reading a log and a person who just lost a save want different things,
so both are written rather than one chosen — noted on `explain_io` along
with the rule, since it was the absence of a stated rule that let four
callers drift.

src/ui/code_editor.rs

@@ -494,7 +494,10 @@ impl Tty7App {
                     Err(e) => {
                         return Err(t_fmt(
                             L10nKey::EditorCantOpen,
-                            &[("path", &path.display().to_string()), ("e", &e.to_string())],
+                            &[
+                                ("path", &path.display().to_string()),
+                                ("e", &crate::ui::host_ops::explain_io(&e)),
+                            ],
                         ));
                     }
                 };
@@ -512,7 +515,10 @@ impl Tty7App {
                     Err(e) => {
                         return Err(t_fmt(
                             L10nKey::EditorCantRead,
-                            &[("path", &path.display().to_string()), ("e", &e.to_string())],
+                            &[
+                                ("path", &path.display().to_string()),
+                                ("e", &crate::ui::host_ops::explain_io(&e)),
+                            ],
                         ));
                     }
                 };