feat: portal-wide device selector — session-based, all pages now use selected device

- _portal_device_id() reads from session, falls back to DEVICE_NAME env var
- GET/POST /api/v1/portal/select-device for selector data + set
- Dropdown in server base.html topbar, JS populates from API, page reloads on change
- All portal pages (overview, trips, dashcam, DTC, device settings) now use selected device

server/templates/base.html

@@ -86,7 +86,12 @@
                     <span class="fw-bold text-dark">Yaris OBD</span>
                     <span class="badge bg-warning text-dark ms-1" style="font-size:.6rem;">Remote</span>
                 </a>
-                <div class="ms-auto d-flex align-items-center gap-2">
+                <div class="d-flex align-items-center gap-2">
+                    <select class="form-select form-select-sm" id="portal-device-selector" style="width:auto;min-width:10rem;" aria-label="Select device">
+                        <option value="">Loading…</option>
+                    </select>
+                </div>
+                <div class="d-flex align-items-center gap-2">
                     <span class="text-muted small d-none d-sm-inline">{{ current_user.username }}</span>
                     <a href="{{ url_for('logout') }}" class="btn btn-outline-secondary btn-sm" title="Logout">
                         <i class="fas fa-sign-out-alt fa-fw"></i>
@@ -136,7 +141,34 @@
     <script src="{{ url_for('static', filename='js/bs-init.js') }}"></script>
     <script src="{{ url_for('static', filename='js/theme.js') }}"></script>
 
-    {# ── Shared confirmation helper ────────────────────────────────────────── #}
+    {# ── Device selector ─────────────────────────────────────────────────── #}
+    <script>
+    (function () {
+        const sel = document.getElementById("portal-device-selector");
+        if (!sel) return;
+        async function load() {
+            const r = await fetch("/api/v1/portal/select-device");
+            if (!r.ok) return;
+            const d = await r.json();
+            sel.innerHTML = d.devices.map(dev =>
+                '<option value="' + dev.id + '"' + (dev.id === d.selected_device_id ? " selected" : "") + ">" +
+                (dev.device_name || dev.name) + (dev.portal_default ? " (default)" : "") +
+                "</option>"
+            ).join("");
+            if (!d.devices.length) sel.innerHTML = '<option value="">No devices</option>';
+        }
+        load();
+        sel.addEventListener("change", async function () {
+            const id = sel.value ? parseInt(sel.value) : null;
+            await fetch("/api/v1/portal/select-device", {
+                method: "POST",
+                headers: {"Content-Type": "application/json"},
+                body: JSON.stringify({device_id: id}),
+            });
+            location.reload();
+        });
+    })();
+    </script>
     <script>
     window.confirmDelete = function(message) {
         return Promise.resolve(window.confirm(message));