feat(trip): one-liner, description & retina cover on trip list + page

Give trips an optional one-liner (header.tagline) and description (markdown
content), surface them where they help, and fix the soft cover image — all
editable from admin.

- U1: header.cover_image blueprint field → pagemediaselect media picker.
- U2: new macros/cover.html.twig — single source for cover resolution
  (author-selected → first journal image → none; missing file falls back)
  and retina rendering (1x/2x cropResize + srcset). Merged resolve+render
  into one macro since Twig macros can't return a Medium object.
- U3: trip-list card renders the one-liner (when set) and the retina cover.
- U4: trip-page in-column header gains the one-liner, an expandable
  description, and a thin banner strip — gated behind a trip_header_extras
  partial flag (default off) so the shared home active-trip view is
  unchanged (R12/KTD4).
- U5: styles for the card/header one-liner, collapsible description
  (max-height preview, not line-clamp, so it holds across paragraphs) and
  the banner, with a mobile banner-height reduction.

Covers R1–R15. Verified with new Playwright specs + full trip/home/maps
regression against an isolated worktree dev server.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01RDS6t8wcpbwKvvrxykVQ5K

themes/intotheeast/css/style.css

@@ -950,6 +950,66 @@ body::after {
     color: var(--color-ink-muted);
 }
 
+/* ── Trip page header extras: one-liner, description, banner ──────────────────── */
+
+.home-trip-tagline {
+    font-size: var(--text-md);
+    line-height: var(--leading-snug);
+    color: var(--color-ink-2);
+    margin: 0 0 var(--space-2);
+}
+
+.trip-header-desc {
+    margin: var(--space-3) 0 var(--space-4);
+}
+
+.trip-header-desc-body {
+    font-size: var(--text-sm);
+    line-height: var(--leading-normal);
+    color: var(--color-ink-2);
+}
+
+.trip-header-desc-body > :first-child { margin-top: 0; }
+.trip-header-desc-body > :last-child { margin-bottom: 0; }
+.trip-header-desc-body p { margin: 0 0 var(--space-2); }
+
+/* Collapsed preview ≈3 lines. Uses max-height (not -webkit-line-clamp) so it
+   holds across the multiple <p> that markdown content renders. */
+.trip-header-desc[data-collapsed="true"] .trip-header-desc-body {
+    max-height: 4.8em;
+    overflow: hidden;
+}
+
+.trip-header-desc-toggle {
+    display: inline-block;
+    margin-top: var(--space-1);
+    padding: 0;
+    background: none;
+    border: none;
+    font: inherit;
+    font-size: var(--text-sm);
+    color: var(--color-accent);
+    cursor: pointer;
+}
+
+.trip-header-desc-toggle:hover { color: var(--color-accent-hover); }
+
+.trip-header-banner {
+    width: 100%;
+    height: 200px;
+    margin: var(--space-4) 0 0;
+    border-radius: var(--radius-md);
+    overflow: hidden;
+    background: var(--color-border);
+}
+
+.trip-header-banner img {
+    width: 100%;
+    height: 100%;
+    object-fit: cover;
+    display: block;
+}
+
 /* ── Trip page filter bar ────────────────────────────────────────────────────── */
 
 .feed-sort-bar {
@@ -1045,6 +1105,7 @@ body::after {
     .home-map-col { position: static; height: 40vh; align-self: stretch; }
     .home-map { height: 40vh; }
     .home-feed-col { padding: var(--space-6) var(--space-5); }
+    .trip-header-banner { height: 130px; }
 }
 
 /* ── Past trips archive ──────────────────────────────────────────────────────── */
@@ -1114,6 +1175,13 @@ body::after {
     margin-bottom: var(--space-2);
 }
 
+.trip-card-tagline {
+    font-size: var(--text-sm);
+    line-height: var(--leading-snug);
+    color: var(--color-ink-muted);
+    margin: 0 0 var(--space-3);
+}
+
 .trip-card-meta {
     display: flex;
     gap: var(--space-4);