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/GlassTuner.tsx

@@ -1,5 +1,6 @@
 import { useEffect, useMemo, useState } from "react";
 import Overlay from "./Overlay";
+import { copyText } from "../lib/clipboard";
 import { usePrefs } from "./PrefsProvider";
 
 /**
@@ -205,11 +206,11 @@ export default function GlassTuner() {
     return lines.length ? `${scope} {\n${lines.join("\n")}\n}` : "/* all values at defaults */";
   }, [vars, palette, themeTick]);
 
-  function copy() {
-    navigator.clipboard?.writeText(cssOut).then(() => {
+  async function copy() {
+    if (await copyText(cssOut)) {
       setCopied(true);
       setTimeout(() => setCopied(false), 1200);
-    });
+    }
   }
 
   function persistOpen(v: boolean) {