fix(share): robust clipboard copy with an iOS-safe fallback

navigator.clipboard.writeText is intermittent on iOS Safari (bug 9: the
download share-link copy 'often does neither'). Add lib/clipboard.ts
copyText(): a gesture-safe execCommand path FIRST (before any await can
spend the transient user activation), async Clipboard API as fallback.
Migrate all five copy call sites to it.

frontend/src/components/Sidebar.tsx

@@ -5,6 +5,7 @@ import { Library, Pencil, Share2, SlidersHorizontal, User } from "lucide-react";
 import { api, type FeedFilters, type Tag } from "../lib/api";
 import { defaultLayout, type PanelLayout } from "../lib/panelLayout";
 import { shareUrl } from "../lib/urlState";
+import { copyText } from "../lib/clipboard";
 import { notify } from "../lib/notifications";
 import { useDebounced } from "../lib/useDebounced";
 import { clearedFilters, useActiveFilters } from "../lib/useActiveFilters";
@@ -125,10 +126,9 @@ export default function Sidebar({
   const active = activeCount > 0;
 
   async function shareView() {
-    try {
-      await navigator.clipboard.writeText(shareUrl(filters));
+    if (await copyText(shareUrl(filters))) {
       notify({ level: "success", message: t("sidebar.shareCopied") });
-    } catch {
+    } else {
       notify({ level: "warning", message: t("sidebar.shareFailed") });
     }
   }