feat(onboarding): improve setup UX with clickable triggers and auto-fill buttons

- Turn the "/setup" text on the welcome screen and fallback state into a clickable link that automatically runs the setup command.
- Add an interactive down-arrow "Use in Chat" button next to copy button on typewriter-generated setup code blocks.
- Programmatically trim the "..." placeholder when inserting API keys, focusing the cursor right after "sk-".
- Implement click-delegation for supported provider spans and raw code elements inside the setup guide to instantly pre-populate the input bar.

static/js/models.js

@@ -553,7 +553,7 @@ export async function refreshModels(force = false) {
       box.appendChild(noModels);
       // No endpoints yet: keep the welcome screen focused on first setup.
       const welcomeSub = document.getElementById('welcome-sub');
-      if (welcomeSub) welcomeSub.innerHTML = 'Type <span style="color:var(--accent,var(--red));font-weight:600">/setup</span> to get started.';
+      if (welcomeSub) welcomeSub.innerHTML = 'Type <span class="setup-trigger-link" style="color:var(--accent,var(--red));font-weight:600;cursor:pointer;text-decoration:underline;" title="Click to launch setup">/setup</span> to get started.';
       const welcomeTip = document.getElementById('welcome-tip');
       if (welcomeTip) welcomeTip.textContent = 'Type /setup, then choose Local models or API.';
     } else {