In-feed Back button + clickable source on the article page

* Back button on feed views: drilling into a tag or source from a card now
  remembers where you came from (a small history stack), and a "← Back" appears
  in the view header to return there — chains of drill-ins included. Top-level
  nav (rail/bottom bar) resets the history.
* Article page: the source name is now a link into that source's in-app feed
  (/?source=<id>); the SPA reads the param on load and opens the source view
  (label falls back to the loaded feed's source name). Completes the
  "cards-only v1" — source is clickable on /a/ too now.

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

frontend/src/routes/+page.svelte

@@ -99,7 +99,7 @@
   );
   let viewLabel = $derived(
     selected === 'today' ? 'Highlights from Today'
-      : selected.startsWith('source:') ? (currentSource?.name ?? 'Source')
+      : selected.startsWith('source:') ? (currentSource?.name ?? feed[0]?.source ?? 'Source')
       : selected === 'latest' ? 'Latest'
         : currentTag ? humanize(currentTag)
           : (currentMood?.label ?? cap(currentTopic?.key) ?? '')
@@ -208,13 +208,26 @@
     return `/api/feed?limit=${PAGE}&offset=${offset}${q ? '&' + q : ''}${exq}`;
   }
 
-  // Clicking a source name opens its publication feed; stash the name for the header.
-  function selectSource(id, name) {
-    currentSource = { id, name };
-    select('source:' + id);
+  // Drill into a tag or source view from a card, remembering where we came from
+  // so the in-feed Back button can return there (chains of drill-ins too).
+  let navStack = $state([]); // [{ key, source }] views to return to
+  function drill(key, source = null) {
+    navStack = [...navStack, { key: selected, source: currentSource }];
+    if (source) currentSource = source;
+    select(key);
+  }
+  function goBack() {
+    const prev = navStack[navStack.length - 1];
+    if (!prev) return;
+    navStack = navStack.slice(0, -1);
+    currentSource = prev.source;
+    select(prev.key);
   }
 
   async function select(key, fresh = false) {
+    // Top-level navigation (nav rail / bottom bar) resets the drill-in history;
+    // only tag/source drill-ins build it up.
+    if (!key.startsWith('tag:') && !key.startsWith('source:')) navStack = [];
     selected = key;
     error = '';
     feedDone = false;
@@ -324,7 +337,13 @@
       topics = (await getJSON('/api/categories')).topics;
       try { lanePool = await getJSON('/api/lanes'); } catch { lanePool = null; }
       try { families = await getJSON('/api/families'); } catch { families = []; }
-      await select('today');
+      // Deep link from an article page's source name (/?source=<id>) → its feed.
+      const srcParam = typeof location !== 'undefined' && new URLSearchParams(location.search).get('source');
+      if (srcParam && /^\d+$/.test(srcParam)) {
+        await select('source:' + srcParam);
+      } else {
+        await select('today');
+      }
     } catch (e) {
       error = 'Could not reach Upbeat Bytes.';
     }
@@ -354,6 +373,12 @@
   {:else}
     {#key selected}
       <header class="view-head rise">
+        {#if navStack.length}
+          <button class="viewback" onclick={goBack} aria-label="Go back">
+            <svg viewBox="0 0 24 24" aria-hidden="true"><path d="M19 12H5M11 6l-6 6 6 6" fill="none" stroke="currentColor" stroke-width="2.1" stroke-linecap="round" stroke-linejoin="round"/></svg>
+            Back
+          </button>
+        {/if}
         <h1>{viewLabel}</h1>
         {#if viewSubtitle}<p class="sub">{viewSubtitle}</p>{/if}
       </header>
@@ -361,11 +386,11 @@
       {#if selected === 'today'}
         {#if brief?.items?.length}
           <section class="rise">
-            <ArticleCard article={heroArticle} hero onaction={applyAction} onreplace={replaceArticle} ontag={(t) => select('tag:' + t)} onsource={selectSource} onview={record} onimageerror={heroImageFailed} />
+            <ArticleCard article={heroArticle} hero onaction={applyAction} onreplace={replaceArticle} ontag={(t) => drill('tag:' + t)} onsource={(id, name) => drill('source:' + id, { id, name })} onview={record} onimageerror={heroImageFailed} />
             {#if restArticles.length}
               <div class="grid rest">
                 {#each restArticles as a (a.id)}
-                  <ArticleCard article={a} thumb onaction={applyAction} onreplace={replaceArticle} ontag={(t) => select('tag:' + t)} onsource={selectSource} onview={record} />
+                  <ArticleCard article={a} thumb onaction={applyAction} onreplace={replaceArticle} ontag={(t) => drill('tag:' + t)} onsource={(id, name) => drill('source:' + id, { id, name })} onview={record} />
                 {/each}
               </div>
             {/if}
@@ -377,7 +402,7 @@
       {:else if feed.length}
         <div class="grid rise">
           {#each feed as a (a.id)}
-            <ArticleCard article={a} thumb onaction={applyAction} onreplace={replaceArticle} ontag={(t) => select('tag:' + t)} onsource={selectSource} onview={record} />
+            <ArticleCard article={a} thumb onaction={applyAction} onreplace={replaceArticle} ontag={(t) => drill('tag:' + t)} onsource={(id, name) => drill('source:' + id, { id, name })} onview={record} />
           {/each}
         </div>
         {#if !feedDone}
@@ -406,7 +431,7 @@
                 <p class="fdesc">{f.description}</p>
                 <div class="chips">
                   {#each tags as t (t.key)}
-                    <button class="chip" class:active={selected === 'tag:' + t.key} onclick={() => select('tag:' + t.key)}>{humanize(t.key)}</button>
+                    <button class="chip" class:active={selected === 'tag:' + t.key} onclick={() => drill('tag:' + t.key)}>{humanize(t.key)}</button>
                   {/each}
                 </div>
               </div>
@@ -424,6 +449,14 @@
   main.container { padding-top: 6px; padding-bottom: 40px; min-height: 60vh; }
 
   .view-head { margin: 18px 0 18px; }
+  .viewback {
+    display: inline-flex; align-items: center; gap: 5px; margin-bottom: 10px;
+    background: none; border: 1px solid var(--line); color: var(--accent-deep);
+    border-radius: 999px; padding: 6px 14px; font-size: 0.85rem; cursor: pointer;
+    transition: border-color 0.14s ease;
+  }
+  .viewback:hover { border-color: var(--accent); }
+  .viewback svg { width: 16px; height: 16px; display: block; }
   .view-head h1 { font-size: clamp(2.1rem, 5.5vw, 2.8rem); line-height: 1.05; text-transform: capitalize; }
   .view-head .sub { margin: 8px 0 0; color: var(--muted); font-size: 1.02rem; }
   .view-head::after {