Images Phase 2: uniform brief tiles — photo or topic-colored placeholder

Per review, mixed photo/no-photo rows read as ragged ("the ones without feel
lacking"). Make every brief rest card carry a banner so the grid is uniform: a
compact 16:9 photo when available, otherwise a calm placeholder tinted by the
card's topic accent color (the same per-topic hue as the accent line) with the
topic word set faint in serif. A failed/blocked image falls back to the same
placeholder, so cards never look broken and heights stay identical. Hero remains
the single large image. Light desaturation on photos; no heavy tint.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

frontend/src/routes/+page.svelte

@@ -350,7 +350,7 @@
             {#if restArticles.length}
               <div class="grid rest">
                 {#each restArticles as a (a.id)}
-                  <ArticleCard article={a} onaction={applyAction} onreplace={replaceArticle} ontag={(t) => select('tag:' + t)} onview={record} />
+                  <ArticleCard article={a} thumb onaction={applyAction} onreplace={replaceArticle} ontag={(t) => select('tag:' + t)} onview={record} />
                 {/each}
               </div>
             {/if}