fix(desktop): keep identity key help dialog readable in dark mode (#2854)

## Problem

With macOS in dark mode (the default for fresh profiles is to follow the
system scheme), the onboarding "What's an identity key?" help dialog
renders
its title in near-white on the always-white textured card, making it
unreadable. The body paragraphs stay readable because they use the fixed
olive `--buzz-onboarding-backup-ink`; only the `text-foreground` title
(and
the close button's hover color) flip with the theme.

## Cause

The dialog's `DialogContent` carries `buzz-onboarding-neutral-theme` but
is
portaled outside the `buzz-startup-shell` subtree, so in dark mode it
matches
`.dark .buzz-onboarding-neutral-theme:not(.buzz-startup-shell)`
(`components.css`), which flips `--foreground` to `0 0% 98%`. The
textured
powder card (`buzz-card-textured`) has no dark variant — it is baked
light —
so the near-white title disappears against it.

## Fix

One attribute: pin the dialog to the light neutral theme with
`data-system-color-scheme="light"`. This is the established pattern for
always-light onboarding dialogs (`HostedCommunityOnboarding.tsx`, the
`CommunityOnboardingFlow.tsx` avatar dialog), and the pinned-light CSS
rule
already exists and out-specifies the dark-mode flip. No new CSS.

## Testing

- Added a dark-mode regression test to
`tests/e2e/identity-key-help.spec.ts`
  (already registered in the Playwright smoke project): emulates
  `prefers-color-scheme: dark`, opens the dialog, and asserts the title
resolves to the pinned light-neutral ink `rgb(23, 23, 23)`. Before the
fix
  it rendered `rgb(250, 250, 250)`.
- Manual repro: macOS appearance set to Dark → fresh profile → machine
  onboarding → click "What's an identity key?".

Before/after screenshots are in the comment below.

Signed-off-by: Michael Pfister <pfista@gmail.com>

desktop/src/features/onboarding/ui/IdentityKeyHelpDialog.tsx

@@ -67,6 +67,7 @@ export function IdentityKeyHelpDialog() {
       <DialogContent
         className="buzz-onboarding-neutral-theme max-w-[47.5rem] -translate-y-5"
         closeButtonClassName={ONBOARDING_INK_ICON_CLASS}
+        data-system-color-scheme="light"
         data-testid="identity-key-help-dialog"
         overlayVariant="transparent"
         surface="textured"

desktop/tests/e2e/identity-key-help.spec.ts

@@ -56,3 +56,38 @@ test("identity key help explains the first-run choice", async ({ page }) => {
     "1",
   );
 });
+
+test("identity key help stays readable when the app resolves dark mode", async ({
+  page,
+}) => {
+  await page.emulateMedia({ colorScheme: "dark" });
+  await installMockBridge(page, undefined, {
+    skipCommunitySeed: true,
+    skipOnboardingSeed: true,
+  });
+  await page.goto("/");
+
+  // Fresh profiles follow the system scheme, so the emulated dark scheme is
+  // the first-run repro: the app resolves the dark theme while onboarding.
+  await expect
+    .poll(() =>
+      page.evaluate(() => document.documentElement.classList.contains("dark")),
+    )
+    .toBe(true);
+
+  const trigger = page.getByTestId("identity-key-help-trigger");
+  await expect(trigger).toHaveCSS("opacity", "1", { timeout: 5000 });
+  await trigger.click();
+
+  const dialog = page.getByTestId("identity-key-help-dialog");
+  await expect(dialog).toBeVisible();
+  await waitForAnimations(page);
+
+  // The textured powder card is baked light in both themes, so the dialog pins
+  // the neutral onboarding theme to its light variant. Without the pin the
+  // dark theme flips --foreground to near-white and the title disappears
+  // against the white card.
+  await expect(
+    dialog.getByRole("heading", { name: "What’s an identity key?" }),
+  ).toHaveCSS("color", "rgb(23, 23, 23)");
+});