Add auto-updater (Phase 1 #4)

- tauri-plugin-updater + tauri-plugin-process registered (Rust + npm)
- Updater endpoint: github.com/hoornet/wrystr/releases/latest/download/latest.json
- Ed25519 signing keypair generated; public key in tauri.conf.json;
  private key added to TAURI_SIGNING_PRIVATE_KEY GitHub secret
- Release workflow passes TAURI_SIGNING_PRIVATE_KEY env var so
  tauri-action signs artifacts and generates latest.json manifest
- useUpdater hook: checks for updates 5 s after startup (silent on
  error); exposes available, version, install(), dismiss()
- UpdateBanner: dismissible top-of-app bar shown when an update is
  available — "Update & restart" downloads, installs, and relaunches

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/hooks/useUpdater.ts

@@ -0,0 +1,64 @@
+import { useEffect, useState } from "react";
+import { check } from "@tauri-apps/plugin-updater";
+import { relaunch } from "@tauri-apps/plugin-process";
+
+interface UpdateState {
+  available: boolean;
+  version: string | null;
+  body: string | null;
+  installing: boolean;
+  error: string | null;
+  install: () => Promise<void>;
+  dismiss: () => void;
+}
+
+export function useUpdater(): UpdateState {
+  const [available, setAvailable] = useState(false);
+  const [version, setVersion] = useState<string | null>(null);
+  const [body, setBody] = useState<string | null>(null);
+  const [installing, setInstalling] = useState(false);
+  const [error, setError] = useState<string | null>(null);
+  const [dismissed, setDismissed] = useState(false);
+
+  useEffect(() => {
+    // Check for updates ~5 s after startup (non-blocking)
+    const t = setTimeout(async () => {
+      try {
+        const update = await check();
+        if (update?.available) {
+          setAvailable(true);
+          setVersion(update.version);
+          setBody(update.body ?? null);
+        }
+      } catch {
+        // Update check failure is silent — network may be unavailable
+      }
+    }, 5000);
+    return () => clearTimeout(t);
+  }, []);
+
+  const install = async () => {
+    setInstalling(true);
+    setError(null);
+    try {
+      const update = await check();
+      if (update?.available) {
+        await update.downloadAndInstall();
+        await relaunch();
+      }
+    } catch (err) {
+      setError(String(err));
+      setInstalling(false);
+    }
+  };
+
+  return {
+    available: available && !dismissed,
+    version,
+    body,
+    installing,
+    error,
+    install,
+    dismiss: () => setDismissed(true),
+  };
+}