fix(desktop): remove Projects overview card fills (#3416)

## Summary

The Projects overview now lets the page surface flow through its metrics
and activity cards instead of stacking filled panels. Borders and hover
feedback remain, preserving grouping and interaction cues without the
heavy nested background.

### Related issue

None found.

### Testing

- `pnpm exec biome check
src/features/projects/ui/ProjectsOverviewPanel.tsx
src/features/projects/ui/ProjectsActivityFeed.tsx
tests/e2e/project-pr-review.spec.ts`
- `pnpm build:e2e`
- Focused Playwright smoke test: `project overview does not paint a
background behind its cards` (passed)
- Relevant desktop pre-push checks passed; the unrelated integration
gate was blocked by a stale local checksum for migration 25

Signed-off-by: Thomas Petersen <thomasp@squareup.com>

desktop/src/features/projects/ui/ProjectsActivityFeed.tsx

@@ -257,9 +257,10 @@ function ActivityCard({
   return (
     <div
       className={cn(
-        "relative block w-full rounded-xl border border-border/60 bg-card text-left transition-colors hover:bg-muted/20",
+        "relative block w-full rounded-xl border border-border/60 bg-transparent text-left transition-colors hover:bg-muted/20",
         compact ? "p-3" : "p-4",
       )}
+      data-testid="projects-activity-card"
     >
       <button
         aria-label={`Open ${item.title} in ${item.target.project.name}`}

desktop/src/features/projects/ui/ProjectsOverviewPanel.tsx

@@ -50,7 +50,8 @@ function StatPill({
 }) {
   return (
     <button
-      className="flex flex-col rounded-lg border border-border/60 bg-card px-3.5 py-3 text-left transition-colors hover:bg-muted/30"
+      className="flex flex-col rounded-lg border border-border/60 bg-transparent px-3.5 py-3 text-left transition-colors hover:bg-muted/30"
+      data-testid="projects-overview-stat"
       onClick={onClick}
       type="button"
     >
@@ -78,7 +79,7 @@ export function ProjectsOverviewPanel({
   const stats = overviewStats(projects, summaries);
 
   return (
-    <section className="-mx-4 mb-4 bg-card">
+    <section className="-mx-4 mb-4" data-testid="projects-overview-panel">
       <div className="grid xl:grid-cols-[minmax(0,1fr)_18rem] 2xl:grid-cols-[minmax(0,1fr)_24rem]">
         <div className="order-1 grid grid-cols-2 gap-2 p-4 pt-0 sm:gap-3 xl:order-none xl:col-start-1 xl:row-start-1 xl:grid-cols-4">
           <StatPill

desktop/tests/e2e/project-pr-review.spec.ts

@@ -910,6 +910,41 @@ test("project overview reports aggregate work-item failures", async ({
   );
 });
 
+test("project overview does not paint a background behind its cards", async ({
+  page,
+}) => {
+  await enableProjectsFeature(page);
+  await installMockBridge(page);
+  await page.goto("/", { waitUntil: "domcontentloaded" });
+  await page.getByTestId("open-projects-view").click();
+
+  await expect(page.getByTestId("projects-overview-panel")).toHaveCSS(
+    "background-color",
+    "rgba(0, 0, 0, 0)",
+  );
+
+  const stats = page.getByTestId("projects-overview-stat");
+  await expect(stats).toHaveCount(4);
+  for (let index = 0; index < 4; index += 1) {
+    await expect(stats.nth(index)).toHaveCSS(
+      "background-color",
+      "rgba(0, 0, 0, 0)",
+    );
+    await expect(stats.nth(index)).toHaveCSS("border-style", "solid");
+  }
+
+  const activityCards = page.getByTestId("projects-activity-card");
+  await expect(activityCards.first()).toBeVisible();
+  const activityCardCount = await activityCards.count();
+  for (let index = 0; index < activityCardCount; index += 1) {
+    await expect(activityCards.nth(index)).toHaveCSS(
+      "background-color",
+      "rgba(0, 0, 0, 0)",
+    );
+    await expect(activityCards.nth(index)).toHaveCSS("border-style", "solid");
+  }
+});
+
 test("project without a checkout offers fetch feedback and dropdown cloning", async ({
   page,
 }) => {