fix(frontend): reader back button pops history instead of pushing (0.60.2)

The reader's "back to manga" link was a naked `<a href="/manga/{id}">`
that pushed a new history entry on every tap, so browser-back kept
ping-ponging between detail and reader instead of walking out to
home:

  home → detail → reader → reader-back (push detail) → detail-back
       (pop reader) → reader-back (push detail) → … loop forever

The fix intercepts left-click and calls `window.history.back()`
directly when there's same-tab history to pop. Middle-click /
cmd-click / right-click pass through so "open in new tab" still
works, and a deep-link / fresh-tab visit where `history.length ===
1` falls through to the href so the button still leads somewhere
useful.

An earlier attempt gated the back on
`document.referrer.startsWith(origin)` — but SvelteKit's SPA
`pushState` doesn't update `document.referrer`, so that check was
always false in practice and the default href fired anyway. The
e2e regression added here uses real link clicks (not
`window.location.href`) so it actually exercises the SPA-nav path
the bug lives on.

Verified: home → detail → reader, then reader-back keeps
`history.length` at 4 (popped, not pushed), and a second tap on the
detail back button walks out to /.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

frontend/src/routes/manga/[id]/chapter/[chapter_id]/+page.svelte

@@ -446,6 +446,28 @@
         progressTimer = setTimeout(flushProgress, 1500);
     }
 
+    /**
+     * Reader back behavior — pop browser history instead of pushing a
+     * fresh entry for `/manga/{id}`. Previously this was a naked
+     * `<a href>` and every tap pushed, so browser-back ping-ponged
+     * between detail and reader instead of walking out to home.
+     *
+     * Just check `history.length > 1` — `document.referrer` does NOT
+     * update across SvelteKit SPA navigations, so referrer-based
+     * gating silently fell back to the default href every time.
+     * Middle-click / cmd-click / right-click are passed through so
+     * "open in new tab" still works.
+     */
+    function onBackClick(e: MouseEvent) {
+        if (!browser) return;
+        if (e.button !== 0 || e.metaKey || e.ctrlKey || e.shiftKey || e.altKey) return;
+        if (window.history.length > 1) {
+            e.preventDefault();
+            window.history.back();
+        }
+        // else: let the href navigate (deep-link / fresh tab path)
+    }
+
     // Single-mode: every page change moves the high-water mark.
     // Intentionally NOT depending on `mode` — toggling layout doesn't
     // change the read position, and re-running this effect on a mode
@@ -552,7 +574,12 @@
 </svelte:head>
 
 <nav class="reader-nav" aria-label="reader" bind:this={readerNavEl}>
-    <a href="/manga/{manga.id}" class="back" data-testid="back-to-manga">
+    <a
+        href="/manga/{manga.id}"
+        class="back"
+        onclick={onBackClick}
+        data-testid="back-to-manga"
+    >
         <ArrowLeft size={18} aria-hidden="true" />
         {#if manga.cover_image_path}
             <img