feat(dashboard): always-visible Tour button — replay welcome modal any time

The 10-step welcome tour was first-run-only (persisted in IndexedDB).
After dismissing, users had no clear path back to it.

Fix:
- Topbar gets a '★ Tour' ghost button next to '?' that fires
  CustomEvent('nv-show-tour') any time.
- Help-center Quickstart adds a primary 'Take the interactive 10-step
  tour' button that closes help and launches the tour.
- nv-help listens for 'nv-show-help-close' to support the help→tour
  hand-off cleanly.

Settings drawer already has 'Replay welcome tour' (added earlier);
this just makes the same action one-click from the always-visible
topbar.

Co-Authored-By: claude-flow <ruv@ruv.net>

dashboard/src/components/nv-topbar.ts

@@ -117,6 +117,11 @@ export class NvTopbar extends LitElement {
         @click=${this.openSeedModal}>
         seed: <b>0x${seedHex}</b>
       </span>
+      <button class="ghost" id="tour-btn" title="Replay the 10-step welcome tour"
+        aria-label="Replay welcome tour"
+        @click=${() => window.dispatchEvent(new CustomEvent('nv-show-tour'))}>
+        ★ Tour
+      </button>
       <button class="ghost" id="help-btn" title="Help (press ? any time)" aria-label="Open help"
         @click=${() => window.dispatchEvent(new CustomEvent('nv-show-help'))}>
         ?