fix(a11y): make the form error color meet WCAG AA in both schemes

Mantine's `--mantine-color-error` defaults to `red-6` in light and `red-8` in
dark. Both fail the 4.5:1 AA threshold that applies at the 12px size it renders
input error text: 4.48:1 on white, and 1.93:1 on our charcoal body. Override to
`red-7` (5.72:1) and `red-4` (6.44:1) — the latter matching the one-to-two-step
lift App.css already applies to the semantic status/log reds for dark.

Surfaced by the JsonObjectInput stories, which are the first in the repo to
render an input error state and so the first to put this in front of the
Storybook a11y gate. It is app-wide, not specific to that component — every
form error in the Inspector was affected.

It has to go through `cssVariablesResolver`: MantineProvider injects these
variables into a `<style>` block appended after the stylesheet imports, so a
`:root` rule in App.css loses to it on source order at equal specificity.

That resolver was duplicated in main.tsx and .storybook/preview.tsx and missing
from renderWithMantine, so the three could drift on any token they set. Extract
one `theme/cssVariables.ts` and use it at all three sites.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_0177mPHAdECD18nLLCTwR5rh
Signed-off-by: cliffhall <cliff@futurescale.com>

clients/web/src/test/renderWithMantine.tsx

@@ -3,6 +3,7 @@ import { act, render, type RenderOptions } from "@testing-library/react";
 import { MantineProvider, type MantineColorScheme } from "@mantine/core";
 import { afterEach, expect, vi } from "vitest";
 import { theme } from "../theme/theme";
+import { cssVariablesResolver } from "../theme/cssVariables";
 
 // Options accepted by both render helpers: the standard RTL options (minus
 // `wrapper`, which we own) plus an optional forced `colorScheme`. The default
@@ -30,7 +31,12 @@ export type MantineTransitionsRenderOptions = MantineRenderOptions & {
 function makeWrapper(env: "test" | "default", colorScheme: MantineColorScheme) {
   return function Wrapper({ children }: { children: ReactNode }) {
     return (
-      <MantineProvider theme={theme} defaultColorScheme={colorScheme} env={env}>
+      <MantineProvider
+        theme={theme}
+        defaultColorScheme={colorScheme}
+        env={env}
+        cssVariablesResolver={cssVariablesResolver}
+      >
         {children}
       </MantineProvider>
     );