feat(matrix): unify Techniques and Coverage Matrix into one page

Techniques (/techniques) and Coverage Matrix (/matrix) showed the same
underlying data (Technique.status_global) with completely different
visual styles — discrete Tailwind status colors + ID+name cells on
Techniques, vs. a continuous score-gradient hex color + ID-only cells
(name via tooltip) on the heatmap. Coverage Matrix also has 3 layers
(Threat Actor, Detection Rules, Campaign) with no Techniques equivalent,
so Techniques was the strict subset — merged into Coverage Matrix rather
than the other way around.

- HeatmapCell now uses the same discrete status-color palette as the old
  TechniqueCell (cellColors.ts), shows the technique name inline, and
  keeps zoom/virtualization/hover-tooltip untouched.
- HeatmapLegend restyled to match (swatch + label + description per
  tier, review_required as an overlay badge) instead of the old
  hex-gradient bar.
- MatrixPage gains a Matrix/List view toggle and a status filter
  (coverage/threat-actor layers only) — both ported from the old
  Techniques page — so nothing it offered is lost.
- /techniques now redirects to /matrix; removed TechniquesPage,
  AttackMatrix, TechniqueCell (fully superseded). /techniques/:mitreId
  (detail) and /techniques/review-queue are untouched.

Known pre-existing limitation carried over, not introduced or fixed
here: the heatmap only places a multi-tactic technique under its first
tactic column (_format_tactic takes tactic_str.split(",")[0]), while the
old Techniques matrix placed it under every tactic it belongs to.

frontend/src/App.tsx

@@ -9,7 +9,6 @@ import LoginPage from "./pages/LoginPage";
 import DashboardPage from "./pages/DashboardPage";
 
 /* ── Lazy loaded (V1-V3 pages) ────────────────────────────────────── */
-const TechniquesPage = React.lazy(() => import("./pages/TechniquesPage"));
 const MatrixPage = React.lazy(() => import("./pages/MatrixPage"));
 const ExecutiveDashboardPage = React.lazy(() => import("./pages/ExecutiveDashboardPage"));
 const CompliancePage = React.lazy(() => import("./pages/CompliancePage"));
@@ -50,7 +49,8 @@ export default function App() {
         {/* ── Core ─────────────────────────────────────────────── */}
         <Route path="/dashboard" element={<DashboardPage />} />
 
-        <Route path="/techniques" element={<Suspense fallback={<LoadingSpinner text="Loading…" />}><TechniquesPage /></Suspense>} />
+        {/* Techniques was merged into the Coverage Matrix's "Coverage" layer + List view */}
+        <Route path="/techniques" element={<Navigate to="/matrix" replace />} />
         <Route path="/techniques/:mitreId" element={<Suspense fallback={<LoadingSpinner text="Loading…" />}><TechniqueDetailPage /></Suspense>} />
 
         <Route path="/matrix" element={<Suspense fallback={<LoadingSpinner text="Loading…" />}><MatrixPage /></Suspense>} />