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/templates/macros/cover.html.twig

@@ -0,0 +1,45 @@
+{#
+  Shared trip cover: one source of truth for cover resolution + retina
+  rendering, used by both the trip-list card (trips.html.twig) and the
+  trip-page banner strip (trip-feed-col.html.twig) so the two surfaces
+  cannot drift (KTD2).
+
+  Resolution (R7 / R11):
+    1. author-selected header.cover_image, when it resolves to page media;
+    2. else the first published journal entry's first image;
+    3. else nothing.
+  A set-but-missing cover_image (deleted/moved file) is `is not defined` in
+  page media, so it falls through to the auto-pick rather than rendering a
+  broken image (R11).
+
+  Twig macros can only emit strings — they cannot return a Medium object —
+  so resolution and rendering live together in one macro: it emits the
+  wrapper + retina <img> when a cover exists, and nothing at all when none
+  does (which gives R9/AE4 a clean text-only header for free).
+
+  Retina (R6 / R14): two explicit cropResize derivatives (1x at w×h, 2x at
+  2w×2h) as an explicit `srcset` with w-descriptors; `alt` is the trip title.
+#}
+{% macro render(trip_page, alt, w, h, wrapper_class, sizes) %}
+{%- set cover = null -%}
+{%- if trip_page.header.cover_image and trip_page.media[trip_page.header.cover_image] is defined -%}
+    {%- set cover = trip_page.media[trip_page.header.cover_image] -%}
+{%- else -%}
+    {%- set dailies_page = grav.pages.find(trip_page.route ~ '/dailies') -%}
+    {%- if dailies_page -%}
+        {%- set first_entry = dailies_page.children.published()|first -%}
+        {%- if first_entry and first_entry.media.images|length > 0 -%}
+            {%- set cover = first_entry.media.images|first -%}
+        {%- endif -%}
+    {%- endif -%}
+{%- endif -%}
+{%- if cover -%}
+<div class="{{ wrapper_class }}">
+    <img src="{{ cover.cropResize(w, h).url }}"
+         srcset="{{ cover.cropResize(w, h).url }} {{ w }}w, {{ cover.cropResize(w * 2, h * 2).url }} {{ (w * 2) }}w"
+         sizes="{{ sizes|default('100vw') }}"
+         alt="{{ alt }}"
+         loading="lazy">
+</div>
+{%- endif -%}
+{% endmacro %}