fire-planner: Wave 2 chart-first — flex spending, categorised life

events, interactive Visx Gantt + spending-profile chart

Charts are now the primary editor for life events. The Plan-tab body
re-orders to make charts ~80% of viewport real-estate; legacy form
sections are collapsed into a drawer.

Backend:
- alembic 0004: life_event.category enum (essential / discretionary /
  not_spending). Defaults to essential so existing rows keep their
  full spending impact.
- Simulator gains discretionary_outflows + flex_rules params. Tracks
  per-path running ATH, applies the deepest applicable cut to
  discretionary outflows when portfolio drops vs ATH (PLab-style flex
  spending). Cut amount stays in the portfolio (refund pattern).
- New flex_spending module with FlexRule + applicable_cut +
  cuts_per_year (vectorised). Sortable rules; "deepest cut wins" so
  users specify cumulative cuts at each tier.
- New /scenarios/{id}/spending-profile endpoint returning per-year
  base / essential / discretionary / flex_cut / total breakdown.
- SimulateRequest gains flex_rules + life_event.category roundtrip.
- 8 new tests; 246 total pytest pass; mypy + ruff clean.

Frontend (Visx + ECharts):
- Installed @visx/{scale,shape,group,axis,event,responsive,tooltip}
  for native SVG drag interactions.
- New <SpendingProfileChart> — Visx stacked-area of base/essential/
  discretionary with red flex-cut overlay, hover tooltip, click-to-
  scrub-year.
- New <EventGantt> — interactive Visx Gantt:
    * Click empty space → popover create at that year (default
      essential spending event)
    * Click a bar → inline edit popover (name, kind, range, £/y,
      category) with delete button
    * Drag bar middle → moves the whole event (year-resolution snap)
    * Drag bar edges → resizes year_start / year_end
    * All gestures persist via PATCH /life-events/{id}
- New <FlexRulesEditor> — list of {from_ath_pct, cut} tiers, save-on-
  change to scenario.config_json.flex_rules.
- Plan-tab redesign: NW fan dominant top with floating stat badges
  (Year/Age/NW/Δ NW/Spending/Eff. tax) over the chart; spending-
  profile chart middle; Gantt bottom; flex-rules editor; legacy form
  sections in a collapsed <details> drawer.
- Frontend typecheck + 7 vitest tests + production build all clean.

frontend/package.json

@@ -14,6 +14,13 @@
   },
   "dependencies": {
     "@tanstack/react-query": "^5.62.0",
+    "@visx/axis": "^3.12.0",
+    "@visx/event": "^3.12.0",
+    "@visx/group": "^3.12.0",
+    "@visx/responsive": "^3.12.0",
+    "@visx/scale": "^3.12.0",
+    "@visx/shape": "^3.12.0",
+    "@visx/tooltip": "^3.12.0",
     "echarts": "^6.0.0",
     "echarts-for-react": "^3.0.2",
     "react": "^19.0.0",
@@ -24,6 +31,7 @@
   },
   "devDependencies": {
     "@tailwindcss/vite": "^4.0.0",
+    "@testing-library/dom": "^10.4.1",
     "@testing-library/jest-dom": "^6.6.0",
     "@testing-library/react": "^16.1.0",
     "@testing-library/user-event": "^14.5.0",