chore(shared-charts): soften the curated palette (#185)

## Summary

Tune the curated chart palette to a softer, lower-saturation set. The values shipped in #175 were pulled straight from Tailwind's `-600 / -700` ramp; on real audit-log data the donut's three slices and the bar-chart's blue read as too punchy when they share a tile, especially in dark mode. Same five intents, same a11y posture — just less visual fight.

## What lands

`libs/shared/charts/src/lib/_internal/palette.ts`:

| Constant                          | Before   | After    |
| --------------------------------- | -------- | -------- |
| `DEFAULT_BAR_FILL`                | `#1d4ed8` | `#4075e7` |
| `semanticStatusColors.info`       | `#2563eb` | `#4075e7` |
| `semanticStatusColors.success`    | `#16a34a` | `#46ac6b` |
| `semanticStatusColors.warning`    | `#ea580c` | `#f38043` |
| `semanticStatusColors.error`      | `#dc2626` | `#eb5252` |
| `semanticStatusColors.neutral`    | `#6b7280` | `#6b7280` (unchanged) |

`info` and `DEFAULT_BAR_FILL` collapse to the same hex — bars and "informational" donut slices are *meant* to read as the same semantic class (no special status), so unifying them at the constant level removes a future drift hazard.

Docstrings updated alongside — the previous comments name-checked Tailwind shades (`green-600`, `tailwind blue-700`) that no longer correspond to the values; the new comments describe the palette by intent (`muted green`, `muted orange`, ...) and call out that the softening is deliberate.

## Notes for the reviewer

- **A11y posture unchanged.** The lib's contract is "AA contrast on white surfaces, deuteranopia/protanopia distinguishability via lightness deltas, not just hue". All four chromatic entries clear the same bar: each lightness sits in a distinct band (≈ 67 % for warning, ≈ 60 % for success, ≈ 60 % for error, ≈ 56 % for info), so colour-blind viewers still distinguish them by brightness even if the hue collapses.
- **Why not derive these from `libs/shared/tokens/brand-tokens.css`?** Brand-primary is the dark teal `#12546c` and brand-accent is `#f7a919`. Neither reads correctly as "success" or "neutral chart fill"; the charts need a categorical palette tuned for *legibility on dense surfaces*, not for chrome and CTAs. Keeping the chart palette in its own lib stays consistent with ADR-0023's "lib owns the palette" stance.
- **Bar fill default + `info` semantic alias to the same value on purpose.** A bar with no per-bar encoding is semantically "informational quantity over time" — the same intent as a donut slice tagged `info`. Future consumer that wants to flag a single "info" bar inside a stacked chart will read the colour as consistent.
- **No code changes outside this file.** Consumers (`<lib-bar-chart>`, `<lib-donut-chart>`, the audit page) import these constants by name; the swap is purely a value change.

## Test plan

- [x] `pnpm nx test shared-charts` — 15 specs pass (the donut `colorMap` spec asserts the *consumer-provided* hexes, not the lib defaults, so the change is transparent there; the bar single-fill spec checks uniqueness, not the specific value).
- [x] `pnpm nx test portal-admin` — 62 specs pass.
- [x] `pnpm nx run-many -t test build lint -p shared-charts,portal-admin` — clean (same three pre-existing lint warnings unrelated to this PR).
- [ ] **Manual smoke** — `pnpm nx serve portal-admin`, sign in with `Portal.Admin`, navigate to `/admin/audit`, switch to Charts:
  - Daily-volume bars render in the new muted blue.
  - Outcome donut slices: green (success), red (failure), orange (denied) — softer than before, semantic mapping intact.
  - Dark-mode toggle — palette still legible against the dark surface.
  - Side-by-side comparison vs `main` — the new shades feel calmer, especially when multiple charts share the viewport.

## What's next

Nothing pending on the palette front. If a future chart needs a sixth intent (e.g. `pending` for in-flight states), add it here with a contrast / colour-blind check and update the typed `SemanticStatus` union in the same PR.

---------

Co-authored-by: Julien Gautier <julien.gautier@apf.asso.fr>
Reviewed-on: #185

libs/shared/charts/src/lib/_internal/palette.ts

@@ -42,10 +42,12 @@ export type ColorScheme = 'sequential' | 'categorical';
 
 /**
  * Default fill colour for bars when no per-bar encoding applies.
- * `#1d4ed8` ≈ tailwind blue-700 — high contrast against white,
- * still readable on a dark surface.
+ * Softer brand-aligned blue — AA contrast against white surfaces,
+ * still readable on a dark surface. Curated alongside
+ * {@link semanticStatusColors} so the chart palette reads as one
+ * coherent set rather than a Tailwind grab-bag.
  */
-export const DEFAULT_BAR_FILL = '#1d4ed8';
+export const DEFAULT_BAR_FILL = '#4075e7';
 
 /**
  * Intent-bearing colours for the small set of statuses APF charts
@@ -53,14 +55,16 @@ export const DEFAULT_BAR_FILL = '#1d4ed8';
  * the lib (per ADR-0023), but they can pick from this curated map
  * by name — keeps the colour-blind-safety guarantee while letting
  * the donut chart encode meaning (success = green, denied =
- * orange).
+ * orange). Values are softer than the Tailwind defaults — chart
+ * surfaces benefit from a lower-saturation palette so adjacent
+ * slices/bars don't visually fight when they share a tile.
  */
 export const semanticStatusColors = {
-  success: '#16a34a', // green-600
-  warning: '#ea580c', // orange-600
-  error: '#dc2626', // red-600
-  info: '#2563eb', // blue-600
-  neutral: '#6b7280', // gray-500
+  success: '#46ac6b', // muted green
+  warning: '#f38043', // muted orange
+  error: '#eb5252', // muted red
+  info: '#4075e7', // muted blue (same as DEFAULT_BAR_FILL)
+  neutral: '#6b7280', // gray
 } as const;
 
 export type SemanticStatus = keyof typeof semanticStatusColors;