feat(transport): swap Anthropic → opencode; add persona switcher

Part 1 — Transport swap:
- Replace anthropic.AsyncAnthropic streaming with httpx SSE client
  calling opencode's OpenAI-compat /v1/chat/completions on sin:4096
- Auth: basic auth opencode:$OPENCODE_PASSWORD
- Env: OPENCODE_URL (default http://sin:4096), OPENCODE_PASSWORD
- Sidecar binding (sin:4098) consulted per message to resolve active
  persona; voice read from binding → cart → env default
- Helper _session_id_for_user: deterministic sha256 slug per email
  so sidecar binding survives WebSocket reconnects
- anthropic dep retained in pyproject.toml (not removed — P4 may use it)

Part 2 — Persona switcher:
- PERSONAS dict: bt7274, friday, samantha (slug → voice/backend/prompt)
- POST /api/persona  — bind persona via sidecar, maps slug → full config
- GET  /api/persona/current — return current binding
- GET  /api/personas — list available personas
- chat.html: persona <select> in topnav with server-rendered active state
- chat.js: onChange → fetch /api/persona, update __personaName +
  status badge + system message in conversation feed

TODO: add CSS polish for .topnav__persona-wrap (inherits base styles for now)

app/static/chat.js

@@ -411,6 +411,53 @@ if ($recal) {
   });
 }
 
+// ---------- persona switcher ----------
+
+const $personaSelect = document.getElementById('persona-select');
+const $personaStatus = document.getElementById('persona-status');
+
+if ($personaSelect) {
+  // Set initial selection from server-rendered bound slug
+  if (window.__boundSlug) {
+    $personaSelect.value = window.__boundSlug;
+  }
+
+  $personaSelect.addEventListener('change', async () => {
+    const slug = $personaSelect.value;
+    if (!slug) return; // "— default —" selected
+
+    $personaSelect.disabled = true;
+    if ($personaStatus) $personaStatus.textContent = 'binding…';
+
+    try {
+      const resp = await fetch('/api/persona', {
+        method: 'POST',
+        headers: { 'Content-Type': 'application/json' },
+        body: JSON.stringify({ slug }),
+      });
+      if (!resp.ok) {
+        const err = await resp.json().catch(() => ({}));
+        throw new Error(err.detail || `HTTP ${resp.status}`);
+      }
+      const data = await resp.json();
+      // Update display name in UI
+      if ($personaStatus) $personaStatus.textContent = data.display || slug;
+      // Update the speaker label for subsequent BT messages
+      if (data.display) window.__personaName = data.display;
+      // Subtle system message in conversation so the switch is visible
+      const body = makeMsg('system');
+      body.textContent = `persona bound → ${data.display || slug}`;
+    } catch (err) {
+      console.warn('persona bind failed:', err.message);
+      if ($personaStatus) $personaStatus.textContent = 'bind failed';
+      // revert select to previous value
+      $personaSelect.value = window.__boundSlug || '';
+    } finally {
+      $personaSelect.disabled = false;
+    }
+  });
+}
+
 // ---------- init ----------
 
 connect();