ui: tidy job-page download into a single dropdown

Replace the floating 'Download log' button + bare '.ndjson' link with
one cohesive dropdown menu — same affordance as the rest of the
header, opens to two well-described options.

- Native <details><summary> for keyboard + no-JS support; only the
  click-outside-to-close handler is JS (a few lines).
- New .dropdown / .dropdown-menu / .dropdown-item tokens in
  web/styles/input.css. Reusable for future header menus
  (host-detail overflow, source-group action menus, etc).
- Chevron flips 180 degrees when open via .dropdown[open] selector.
- Each option has a label + a mono hint line explaining when to pick it
  (.txt for humans / paste into a ticket; .ndjson for jq / tooling).

web/styles/input.css

@@ -206,6 +206,59 @@
   .src-row.clickable > .row-link { pointer-events: auto; }
   .src-row.clickable > .row-action { pointer-events: auto; }
 
+  /* ---------- dropdown menu (header actions) ----------
+   * Uses native <details><summary> for keyboard + no-JS support.
+   * The summary is styled like a .btn, the panel sits absolute below.
+   * Click-outside-to-close handled by CSS via :has() — no JS.
+   */
+  .dropdown { position: relative; display: inline-block; }
+  .dropdown summary {
+    list-style: none; cursor: pointer;
+    /* match .btn shape */
+    font-size: 12px; font-weight: 500;
+    padding: 6px 11px; border-radius: 5px;
+    background: transparent;
+    border: 1px solid var(--line);
+    color: var(--ink-mid);
+    transition: all 120ms ease;
+    display: inline-flex; align-items: center; gap: 6px;
+    user-select: none;
+  }
+  .dropdown summary::-webkit-details-marker { display: none; }
+  .dropdown summary::marker { content: ""; }
+  .dropdown summary:hover { background: var(--panel-hi); color: var(--ink); }
+  .dropdown summary .chev {
+    font-size: 9px; color: var(--ink-fade);
+    transition: transform 120ms ease;
+  }
+  .dropdown[open] summary .chev { transform: rotate(180deg); }
+  .dropdown[open] summary { background: var(--panel-hi); color: var(--ink); }
+  .dropdown-menu {
+    position: absolute; top: calc(100% + 4px); right: 0;
+    z-index: 30;
+    min-width: 220px;
+    background: var(--panel);
+    border: 1px solid var(--line);
+    border-radius: 6px;
+    box-shadow: 0 6px 24px -8px rgba(0,0,0,0.55);
+    padding: 4px;
+  }
+  .dropdown-item {
+    display: block;
+    padding: 8px 11px;
+    border-radius: 4px;
+    text-decoration: none;
+    color: var(--ink-mid);
+    font-size: 12.5px;
+    line-height: 1.35;
+  }
+  .dropdown-item:hover { background: var(--panel-hi); color: var(--ink); }
+  .dropdown-item .label { display: block; color: var(--ink); font-weight: 500; }
+  .dropdown-item .hint  {
+    display: block; font-size: 11px; color: var(--ink-mute); margin-top: 2px;
+    font-family: 'JetBrains Mono', ui-monospace, monospace;
+  }
+
   /* ---------- snapshot picker rows (Restore wizard step 1) ---------- */
   .snap-row {
     display: grid; align-items: center;