fix: cursor desync in highlighted textarea (#68) + update indicator (#67)

- Remove font-style:italic from .ht-italic (different glyph widths cause desync)
- Remove font-weight:600 from .ht-heading (bolder glyphs are wider)
- Remove background/border-radius from code token spans
- Remove text-decoration from .ht-link
- Token spans now ONLY use color — zero text geometry changes
- Use inherit for font-size/line-height in shared .ht-backdrop/.ht-textarea
- Add update check hook: polls GitHub releases, shows indicator in sidebar

src/hooks/useUpdateCheck.ts

@@ -0,0 +1,65 @@
+import { useState, useEffect } from 'react';
+
+const GITHUB_API = 'https://api.github.com/repos/MarlBurroW/pinchchat/releases/latest';
+const CHECK_INTERVAL = 60 * 60 * 1000; // 1 hour
+const CACHE_KEY = 'pinchchat-latest-version';
+
+interface UpdateInfo {
+  available: boolean;
+  latestVersion: string | null;
+  releaseUrl: string | null;
+}
+
+export function useUpdateCheck(currentVersion: string): UpdateInfo {
+  const [info, setInfo] = useState<UpdateInfo>({ available: false, latestVersion: null, releaseUrl: null });
+
+  useEffect(() => {
+    let timeout: ReturnType<typeof setTimeout>;
+
+    async function check() {
+      try {
+        // Check cache first
+        const cached = localStorage.getItem(CACHE_KEY);
+        if (cached) {
+          const { version, url, ts } = JSON.parse(cached);
+          if (Date.now() - ts < CHECK_INTERVAL) {
+            if (version && isNewer(version, currentVersion)) {
+              setInfo({ available: true, latestVersion: version, releaseUrl: url });
+            }
+            timeout = setTimeout(check, CHECK_INTERVAL - (Date.now() - ts));
+            return;
+          }
+        }
+
+        const res = await fetch(GITHUB_API, { headers: { Accept: 'application/vnd.github.v3+json' } });
+        if (!res.ok) return;
+        const data = await res.json();
+        const tag: string = data.tag_name?.replace(/^v/, '') || '';
+        const url: string = data.html_url || '';
+
+        localStorage.setItem(CACHE_KEY, JSON.stringify({ version: tag, url, ts: Date.now() }));
+
+        if (tag && isNewer(tag, currentVersion)) {
+          setInfo({ available: true, latestVersion: tag, releaseUrl: url });
+        }
+      } catch { /* silent */ }
+      timeout = setTimeout(check, CHECK_INTERVAL);
+    }
+
+    check();
+    return () => clearTimeout(timeout);
+  }, [currentVersion]);
+
+  return info;
+}
+
+/** True if remote is newer than local (semver compare) */
+function isNewer(remote: string, local: string): boolean {
+  const r = remote.split('.').map(Number);
+  const l = local.split('.').map(Number);
+  for (let i = 0; i < 3; i++) {
+    if ((r[i] || 0) > (l[i] || 0)) return true;
+    if ((r[i] || 0) < (l[i] || 0)) return false;
+  }
+  return false;
+}