style: clean up dead CSS, add CSS variables, improve responsive rules (Task 10.10)

- Remove dead .kpi-row/.kpi-card CSS (KPI row replaced by header fraction KPIs)
- Remove dead .top-header__breadcrumb CSS (breadcrumb removed in 10.3)
- Remove dead .filter-select CSS (replaced by dcc.Dropdown)
- Remove dead .sidebar__item--disabled CSS (disabled items removed in restructure)
- Delete unused dash_app/components/kpi_row.py
- Add --sub-header-h and --header-total-h CSS variables
- Update .main and .sub-header to use CSS variables instead of hardcoded values
- Add responsive rules: header KPIs collapse at 1200px, hide at 768px
- Add responsive pathway-filters wrap at 768px
- Remove dead .kpi-row responsive rules

dash_app/components/kpi_row.py

@@ -1,46 +0,0 @@
-"""KPI row component — 4 metric cards with callback-updatable values."""
-from dash import html
-
-
-def make_kpi_row():
-    """Return a section with 4 KPI cards matching 01_nhs_classic.html structure."""
-    return html.Section(
-        className="kpi-row",
-        **{"aria-label": "Key performance indicators"},
-        children=[
-            _kpi_card("Unique Patients", "kpi-patients", "—", "across all trusts"),
-            _kpi_card("Drug Types", "kpi-drugs", "—", "high-cost drugs tracked"),
-            _kpi_card("Total Cost", "kpi-cost", "—", "current period spend"),
-            _kpi_card(
-                "Indication Match",
-                "kpi-match",
-                "—",
-                "GP diagnosis confirmed",
-                modifier="kpi-card--green",
-            ),
-        ],
-    )
-
-
-def _kpi_card(label, value_id, default_value, sub_text, modifier=None):
-    """Build a single KPI card.
-
-    Args:
-        label: uppercase label text
-        value_id: HTML id for the value span (for callback Output)
-        default_value: initial display value before callbacks fire
-        sub_text: description below the value
-        modifier: optional CSS modifier class (e.g. 'kpi-card--green')
-    """
-    card_class = "kpi-card"
-    if modifier:
-        card_class += f" {modifier}"
-
-    return html.Div(
-        className=card_class,
-        children=[
-            html.Div(label, className="kpi-card__label"),
-            html.Div(default_value, className="kpi-card__value", id=value_id),
-            html.Div(sub_text, className="kpi-card__sub"),
-        ],
-    )