fix(chat): tolerate 500 on empty inbox edges; fix(agent): clear LLM-unavailable msg

- chatApi.listThreads: catch on the edges query so a fresh empty
  inbox doesn't bubble a 500 to the UI. New users see 'No
  conversations yet' instead of the error banner.
- agentTools.llmFallback: when the LLM gateway returns 503/500,
  show a clear 'plain-language replies temporarily unavailable'
  message naming the reason, listing the deterministic capabilities
  that still work. No more silent null returns.

qa/_studio_e2e.mjs

@@ -0,0 +1,77 @@
+// End-to-end Studio flow against LIVE.
+import { chromium } from "playwright";
+
+const URL = "https://canvas.flow-master.ai/";
+const calls = [];
+const consoleErrs = [];
+
+const b = await chromium.launch({ headless: true });
+const ctx = await b.newContext({ viewport: { width: 1440, height: 900 } });
+const p = await ctx.newPage();
+p.on("response", (r) => {
+  const u = r.url();
+  if (u.includes("/api/")) calls.push({ status: r.status(), method: r.request().method(), url: u.replace("https://canvas.flow-master.ai", "") });
+});
+p.on("console", (m) => { if (m.type() === "error") consoleErrs.push(m.text().slice(0, 200)); });
+await ctx.addInitScript(() => localStorage.setItem("fm.canvas.tour.seen", String(Date.now())));
+
+const log = (s) => console.log(`[${(Date.now() - t0) / 1000}s] ${s}`);
+const t0 = Date.now();
+
+log("login");
+await p.goto(URL, { waitUntil: "domcontentloaded" });
+await p.waitForTimeout(2500);
+await p.locator(".persona-chip").filter({ hasText: /Mariana/ }).click();
+await p.waitForSelector(".topbar", { timeout: 30000 });
+log("logged in, on " + (await p.evaluate(() => document.querySelector("[class*='shell-']")?.className)));
+
+log("nav to studio");
+const studioTab = p.locator(".tab").filter({ hasText: /Studio/i }).first();
+await studioTab.click({ timeout: 10000 });
+await p.waitForTimeout(2500);
+log("studio scene: " + (await p.evaluate(() => document.querySelector(".scene")?.innerText?.slice(0, 150))));
+
+log("fill intake");
+const desc = p.locator("textarea").first();
+await desc.fill("E2E test: when a store manager requests a laptop, run procurement approval.");
+await p.waitForTimeout(500);
+
+log("click Start drafting");
+await p.locator("button.btn-primary").filter({ hasText: /draft/i }).first().click({ timeout: 8000 });
+await p.waitForTimeout(7000);
+
+log("step now: " + (await p.evaluate(() => {
+  const t = document.querySelector(".scene")?.innerText || "";
+  return t.split("\n").slice(0, 3).join(" | ");
+})));
+
+log("click Confirm structure");
+const confirmBtn = p.locator("button.btn-primary").filter({ hasText: /Confirm structure/i }).first();
+if (await confirmBtn.count()) {
+  await confirmBtn.click({ timeout: 8000 });
+  log("waiting for toast or step change…");
+  await p.waitForTimeout(10000);
+  const toasts = await p.locator(".toaster, .toast, .toast-msg").allTextContents();
+  log("toasts: " + JSON.stringify(toasts.slice(-3)));
+  const step = await p.evaluate(() => {
+    const t = document.querySelector(".scene")?.innerText || "";
+    return t.split("\n").slice(0, 3).join(" | ");
+  });
+  log("now: " + step);
+}
+
+await p.screenshot({ path: "/tmp/canvas-evidence/studio-e2e.png" });
+
+await b.close();
+console.log("\n=== Network ===");
+const byStatus = {};
+for (const c of calls) byStatus[c.status] = (byStatus[c.status] || 0) + 1;
+console.log("by status:", JSON.stringify(byStatus));
+const fails = calls.filter(c => c.status >= 400);
+console.log("\nFails:");
+for (const f of fails) console.log(`  ${f.status} ${f.method} ${f.url.slice(0, 90)}`);
+const applyBatchCalls = calls.filter(c => c.url.includes("apply-batch"));
+console.log("\napply-batch calls:");
+for (const c of applyBatchCalls) console.log(`  ${c.status} ${c.method} ${c.url}`);
+console.log("\nConsole errors:");
+for (const e of consoleErrs.slice(-10)) console.log("  " + e);