fix(share): mobile share chrome and tabbar poster entry

Use a bottom sheet share panel on small screens and add a one-tap
poster button to the single-post footer tabbar extend.

assets/css/frontend/single-post/author-actions.css

@@ -335,6 +335,59 @@ html.dark .share-item {
 .theme-dark .share-item[data-platform="copy"]:hover { background: rgba(255,255,255,0.06); color: #ccc; }
 .theme-dark .share-item[data-platform="poster"]:hover { background: rgba(245,158,11,0.12); color: #f59e0b; }
 
+/* Narrow screens: bottom sheet so dropdown never clips off-screen */
+@media (max-width: 767px) {
+  .share-dropdown {
+    position: fixed;
+    left: 0;
+    right: 0;
+    bottom: 0;
+    top: auto;
+    transform: none;
+    width: 100%;
+    max-width: none;
+    z-index: 1200;
+    padding: 0 0 calc(12px + env(safe-area-inset-bottom, 0px));
+    background: transparent;
+  }
+  .share-dropdown[aria-hidden="false"] {
+    animation: shareSheetIn 0.22s ease-out forwards;
+  }
+  @keyframes shareSheetIn {
+    from { opacity: 0; transform: translateY(16px); }
+    to { opacity: 1; transform: translateY(0); }
+  }
+  .share-dropdown__grid {
+    justify-content: space-around;
+    flex-wrap: wrap;
+    gap: 6px;
+    margin: 0 10px;
+    padding: 14px 12px 16px;
+    border-radius: 14px 14px 10px 10px;
+    box-shadow: 0 -6px 28px rgba(0, 0, 0, 0.16);
+  }
+  .share-item {
+    min-width: 64px;
+    flex: 1 1 28%;
+    max-width: 33%;
+  }
+  #share-qrcode-box {
+    position: static;
+    margin: 10px auto 0;
+    transform: none;
+  }
+  body.aksrt-share-sheet-open {
+    overflow: hidden;
+  }
+  body.aksrt-share-sheet-open::before {
+    content: '';
+    position: fixed;
+    inset: 0;
+    background: rgba(0, 0, 0, 0.35);
+    z-index: 1190;
+  }
+}
+
 .theme-dark .post-drop-meta__menu,
 html.dark .post-drop-meta__menu {
     background: var(--surface-1, #323335);