feat: add global filter sub-header bar (Task 10.4)

Extract chart type toggle and date filter dropdowns from filter_bar.py
into a new sub-header component. Sub-header is fixed-position below the
main header, visible across both views. Filter bar now contains only
drug/trust/directorate buttons for Patient Pathways view.

dash_app/assets/nhs.css

@@ -155,12 +155,47 @@ body {
     font-size: 12px; color: var(--nhs-mid-grey);
 }
 
+/* ── Global Filter Sub-Header ── */
+.sub-header {
+    position: fixed;
+    top: 56px;
+    left: var(--sidebar-w);
+    right: 0;
+    z-index: 150;
+    height: 44px;
+    background: #E8F0FE;
+    border-bottom: 1px solid #C5D4E8;
+    display: flex;
+    align-items: center;
+    padding: 0 24px;
+    gap: 16px;
+}
+.sub-header__group {
+    display: flex;
+    align-items: center;
+    gap: 8px;
+}
+.sub-header__label {
+    font-size: 11px;
+    font-weight: 700;
+    text-transform: uppercase;
+    letter-spacing: 0.06em;
+    color: var(--nhs-dark-blue);
+    white-space: nowrap;
+    opacity: 0.6;
+}
+.sub-header__divider {
+    width: 1px;
+    height: 24px;
+    background: rgba(0, 48, 135, 0.15);
+}
+
 /* ── Main Content ── */
 .main {
     margin-left: var(--sidebar-w);
-    margin-top: 56px;
+    margin-top: 100px;
     padding: 24px;
-    min-height: calc(100vh - 56px);
+    min-height: calc(100vh - 100px);
     display: flex; flex-direction: column; gap: 20px;
 }