UI: persistent upgrade banner; surface render errors

Two bugs reported in 0.1.16 testing:

1. Library button shows a toast "Library is a paid feature ... Tap
   upgrade to unlock" but no Upgrade button is visible anywhere on the
   page. The free-tier banner only rendered when isLicensed() was false,
   so a licensed-but-missing-entitlements user got the toast directing
   them at an invisible button.

2. Clicking the gear (Settings) icon does nothing — the modal never
   appears. The Activity Log button works on the same page.

Fix 1: replace the unlicensed-only renderFreeBanner with renderUpgrade
       Banner driven by shouldShowUpgradeBanner() = !isProTier(). Copy
       adapts by tier:
         • Free: "Free mode · one video at a time · no library, no subs"
         • Limited license (missing core entitlements): "Limited license
           — your license is missing some Core features"
         • Core tier: "Core tier — upgrade to Pro for subs, auto-queue,
           clips"
       Pro tier shows nothing.

Fix 1b: rewordtoasts on Library / Subscribe clicks to drop the
        "tap Upgrade" instruction — the persistent banner is the action
        path now, no need to direct users at a maybe-not-there button.

Fix 2 (diagnostic): wrap the main app.innerHTML build in try/catch.
       A thrown exception in any of the ${...} template substitutions
       silently aborts the innerHTML assignment, leaving the previous
       DOM in place — which looks exactly like a button doing nothing
       when it actually fired and called render(). Now an exception
       lands in a visible error-box with the message + console trace,
       so the next reproduction tells us what's actually throwing.

public/index.html

@@ -1360,7 +1360,7 @@
       // free-tier users, surface the upgrade prompt instead of opening an
       // empty sidebar.
       if (!hasEntitlement("history")) {
-        showToast("Library is a paid feature — keep every summary you process. Tap upgrade to unlock.", "📚");
+        showToast("Library requires a paid license — every summary you process gets saved.", "📚");
         return;
       }
       toggleHistory();
@@ -1369,7 +1369,7 @@
     function handleSubscribeClick() {
       // Subscriptions / channel auto-queue is Pro-only.
       if (!hasEntitlement("subscriptions")) {
-        showToast("Channel & podcast subscriptions are a Pro feature. Upgrade to auto-summarize new uploads.", "📡");
+        showToast("Channel & podcast subscriptions are a Pro-tier feature.", "📡");
         return;
       }
       addSubscriptionFromInput();
@@ -1666,10 +1666,37 @@
       render();
     }
 
-    function renderFreeBanner() {
+    // True for anyone who isn't on the top tier — drives whether to show
+    // the persistent upgrade banner. Covers: unlicensed, unlicensed-skipped,
+    // Core (full or partial), and any "licensed but missing entitlements"
+    // anomaly. Pro users see no banner.
+    function shouldShowUpgradeBanner() {
+      if (!state.license || !state.license.loaded) return false;
+      return !isProTier();
+    }
+
+    function renderUpgradeBanner() {
       const buyUrl = upgradeToProUrl();
+      const free = !isLicensed();
+      const partialCore = isLicensed() && (!hasEntitlement("history") || !hasEntitlement("library"));
+      const fullCore = isLicensed() && hasEntitlement("history") && hasEntitlement("library") && !isProTier();
+
+      let label, descr;
+      if (free) {
+        label = "Free mode";
+        descr = "one video at a time · no library, no subscriptions";
+      } else if (partialCore) {
+        label = "Limited license";
+        descr = "your license is missing some Core features — contact the seller or upgrade";
+      } else if (fullCore) {
+        label = "Core tier";
+        descr = "upgrade to Pro for channel & podcast subscriptions, auto-queue, and clips";
+      } else {
+        return ""; // shouldn't reach
+      }
+
       return `
-        <div class="free-banner" style="
+        <div class="upgrade-banner" style="
           margin: 8px 0 12px;
           padding: 10px 14px;
           background: linear-gradient(90deg, rgba(168,85,247,0.12), rgba(99,102,241,0.10));
@@ -1683,18 +1710,19 @@
           font-size: 13px;
         ">
           <span style="flex:1; min-width: 220px;">
-            <strong style="color:#c4b5fd;">Free mode</strong>
-            &middot; one video at a time &middot;
-            no library, no subscriptions
+            <strong style="color:#c4b5fd;">${label}</strong>
+            &middot; ${descr}
           </span>
           <a href="${escHtml(buyUrl)}" target="_blank" rel="noopener"
              style="background:#a855f7;color:#fff;border:none;padding:6px 12px;border-radius:6px;text-decoration:none;font-size:12px;font-weight:600;">
             Upgrade
           </a>
-          <button onclick="showActivationScreen()"
-             style="background:transparent;color:#94a3b8;border:1px solid #334155;padding:6px 10px;border-radius:6px;cursor:pointer;font-size:12px;">
-            I have a key
-          </button>
+          ${free ? `
+            <button onclick="showActivationScreen()"
+               style="background:transparent;color:#94a3b8;border:1px solid #334155;padding:6px 10px;border-radius:6px;cursor:pointer;font-size:12px;">
+              I have a key
+            </button>
+          ` : ""}
         </div>
       `;
     }
@@ -1741,7 +1769,9 @@
       // a server-side key set via the StartOS config action.
       const submitDisabled = !state.url.trim()
         || (!isSubscribeUrl(state.url) && !state.apiKey.trim() && !state.hasServerKey);
-      app.innerHTML = `
+      let __renderedHtml;
+      try {
+        __renderedHtml = `
         <!-- Top bar: title + action icons -->
         <div class="top-bar">
           <div class="top-left-actions">
@@ -1835,7 +1865,7 @@
         <!-- Settings modal -->
         ${state.settingsOpen ? renderSettingsModal() : ""}
 
-        ${free ? renderFreeBanner() : ""}
+        ${shouldShowUpgradeBanner() ? renderUpgradeBanner() : ""}
 
         ${isSubscribeUrl(state.url) ? `<div id="subscribe-prompt">${renderSubscribePrompt()}</div>` : ""}
         ${state.queue.length > 0 ? renderQueue() : ""}
@@ -1852,6 +1882,21 @@
         ${state.historyOpen ? renderHistorySidebar() : ""}
         ${state.clipPanelOpen ? renderClipPanel() : ""}
       `;
+        app.innerHTML = __renderedHtml;
+      } catch (renderErr) {
+        // A thrown exception inside any ${...} expression silently aborts
+        // the innerHTML assignment, leaving the previous DOM in place — so
+        // a button click that calls render() looks like a no-op. Surface
+        // the actual error instead of leaving the user wondering.
+        console.error("[render] failed to build HTML:", renderErr);
+        app.innerHTML = `<div class="error-box" style="margin:20px;">
+          <strong>Render error:</strong> ${escHtml(renderErr && renderErr.message || String(renderErr))}
+          <div style="font-size:11px;margin-top:6px;opacity:0.7;">
+            Check the browser console for the full stack trace, or share it with the developer.
+          </div>
+        </div>`;
+        return;
+      }
       // Re-init player if the yt-player div exists
       if (state.videoId && ytReady && document.getElementById("yt-player")) {
         setTimeout(() => initPlayer(state.videoId), 50);