Web dashboard step 5: settings page with toml_edit writer

Schema derived from Config::default() and the live config, help text for every
shipped key, env locks and presence-only secrets, typed saves that preserve
comments and validate through Config::load before an atomic rename, provider
add/remove, the change history, and config reload on mtime.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01NHyYupFdBiR4VfoUM7NjSM

src/web/static/app.js

@@ -39,3 +39,11 @@ document.querySelectorAll("details[id]").forEach((details) => {
     details.addEventListener("toggle", () => localStorage.setItem(key, details.open ? "open" : "closed"));
   } catch (_) {}
 });
+/* step 5: settings — "reset to default" fills the field with its default */
+document.addEventListener("click", (event) => {
+  const button = event.target.closest("button[data-reset]");
+  if (!button) return;
+  const input = document.getElementById(button.dataset.reset);
+  if (!input) return;
+  input.value = button.dataset.default;
+});