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/blueprints/trip.yaml

@@ -38,10 +38,9 @@ form:
               help: 'Leave blank if trip is ongoing'
 
             header.cover_image:
-              type: text
-              label: 'Cover Image Filename'
-              placeholder: 'cover.jpg'
-              help: 'Used in the trips listing page'
+              type: pagemediaselect
+              label: 'Cover Image'
+              help: 'Pick from images uploaded to this trip page. Shown on the trips listing and the trip-page banner. Falls back to the first journal entry photo if left unset.'
 
             header.album_url:
               type: text