fix(deps): override postcss ^8.5.16 (GHSA-qx2v-qp2m-jg93) (#170)

* fix(deps): remove postcss devDep + override ^8.5.16 for nested next 8.4.31 (GHSA-qx2v-qp2m-jg93)

postcss is not imported directly — postcss.config.mjs uses types only,
@tailwindcss/postcss bundles its own, Next ships a nested 8.4.31.
npm override forces the transitive version across all install contexts.

Closes #156.

* docs(adr): add ADR-001 recharts 3 upgrade decision + un-ignore docs/decisions/ADR-*.md

---------

Co-authored-by: Emiya Kiritsugu <emiyakiritsugu3@users.noreply.github.com>

Author: EmiyaKiritsugu3

.gitignore

@@ -129,6 +129,7 @@ docs/.obsidian/
 docs/archive/
 docs/assets/
 docs/decisions/
+!docs/decisions/ADR-*.md
 docs/observability/
 docs/operations/
 docs/pdr/

docs/decisions/ADR-001-recharts3-upgrade.md

@@ -0,0 +1,51 @@
+# ADR-001: Recharts 2 → 3 Upgrade
+
+**Status:** Accepted
+
+**Date:** 2026-07-04
+
+## Context
+
+The project used recharts 2.15.4. PR #78 (2026-04-22) upgraded to recharts ^3.8.1 to unblock Tailwind CSS v4 (recharts 3 requires React 19 / Tailwind v4 compatible peer deps). The breaking change was already absorbed in the codebase.
+
+### Breaking change absorbed
+
+Recharts 3 removed `payload`, `label`, and coordinate props from the `ComponentProps` type (used by `Tooltip`, `Legend`, and `Customized` components). The types were moved to named exports `DefaultLegendContentProps` and `TooltipContentProps`.
+
+## Decision
+
+Upgrade `recharts` from 2.15.4 to ^3.8.1 with the import type pivot:
+
+```typescript
+// Before (recharts 2)
+import type { ContentType } from 'recharts/types/component/DefaultLegendContent';
+
+// After (recharts 3)
+import type { DefaultLegendContentProps, TooltipContentProps } from 'recharts';
+```
+
+The import path change is the only migration cost. No runtime API changes affected the project's chart usage.
+
+## Consequences
+
+### Positive
+
+- React 19 / Tailwind v4 compatibility via recharts 3 peer deps
+- `types/component/` deep path removed from sensitive type imports
+- TypeScript strict mode compatible (no `@ts-ignore` needed)
+
+### Negative
+
+- None. The breaking type change was fully absorbed in `src/components/ui/chart.tsx:5`.
+
+## Files Changed
+
+- `src/components/ui/chart.tsx` — recharts import type pivot
+- `package.json` — `recharts` dep bumped from 2.15.4 to ^3.8.1
+
+## Testing
+
+- `npm run typecheck` — 0 errors
+- `npm run test` — 1103/1103
+- `npm run e2e` — 21/21 (dashboard smoke + chart render)
+- `/dashboard` — manual smoke: charts render with correct axis labels, tooltip on hover, legend visible