Add ntfy Basic auth, sign-out button, config-only credentials

- ntfy client supports HTTP Basic auth (username/password) for servers with
  access control; [ntfy] config gains username/password. Scheduler + Test Ntfy
  use it instead of the old settings-only bearer token.
- Settings page no longer has credential inputs (Apify/eBay/ntfy/Resend); they
  are managed in config.toml. Read-only status + Test buttons remain.
- Sidebar gains a Sign out button; in forward-auth mode it routes through the
  Authentik outpost sign_out so the IdP session is cleared.

static/css/app.css

@@ -304,6 +304,23 @@ a.v-feed-name:hover { color: var(--accent); text-decoration: none; }
 }
 .v-side-nav a:hover { color: white; }
 
+/* Sign-out button, styled to read like a nav link (it lives in a <form>). */
+.v-side-signout {
+  display: flex;
+  align-items: center;
+  gap: 0.6rem;
+  width: 100%;
+  padding: 0.7rem 1rem;
+  color: var(--text-2);
+  background: transparent;
+  border: 0;
+  border-left: 3px solid transparent;
+  text-align: left;
+  cursor: pointer;
+  font: inherit;
+}
+.v-side-signout:hover { color: white; }
+
 /* The brand wordmark at the top of the sidebar is also an anchor (→ /), but
    shouldn't pick up the active-item border-left / padding treatment that
    the nav links get. Higher specificity overrides .v-side-nav a defaults. */