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  AttackMatrix, TechniqueCell (fully superseded).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/api/heatmap.ts

@@ -9,6 +9,10 @@ export interface HeatmapMetadata {
 
 export interface HeatmapTechnique {
   techniqueID: string;
+  /** Technique display name — always present now, shown inline on cells (matches the Techniques page's style). */
+  name?: string;
+  /** Real TechniqueStatus, when the layer has one (coverage, threat-actor) — used to color cells the same way the Techniques page does. Absent for detection-rules/campaign layers. */
+  status?: string;
   tactic: string;
   color: string;
   score: number;