feat(desktop): show selected community in rail (#5000)

## Summary
- add a persistent vertical pill beside the active community
- keep the selected state visually distinct from unread dots and mention
badges
- preserve the existing `aria-current` selection semantics

## Screenshot
![Selected community
indicator](https://d24qwcpro867f5.cloudfront.net/repos/block/buzz/prs/5000/selected-community-indicator-v2.png)

## Test plan
- `pnpm exec biome check src/features/sidebar/ui/CommunityRail.tsx
tests/e2e/community-rail.spec.ts`
- `pnpm test` (4,387 passed)
- `pnpm build:e2e && pnpm exec playwright test
tests/e2e/community-rail.spec.ts --project=smoke` (20 passed)
- pre-push hooks: desktop check and 4,387 desktop tests passed on
`c1e80c66d12f73c4eb5c03a19e932439b14caf2d`

Signed-off-by: Wes <wesbillman@users.noreply.github.com>
Co-authored-by: Carl <c7ebe626f000404285d3686e1dc74cc07cc60a9754a150041ba132e14bd3e2ec@buzz.block.builderlab.xyz>

desktop/src/features/sidebar/ui/CommunityRail.tsx

@@ -133,10 +133,17 @@ function CommunityButton({
               {...dragAttributes}
               {...dragListeners}
             >
+              {isActive ? (
+                <span
+                  aria-hidden="true"
+                  className="absolute -left-2.5 h-5 w-1 rounded-r-full bg-primary"
+                  data-testid={`community-rail-active-${community.id}`}
+                />
+              ) : null}
               <span
                 className={cn(
                   "flex h-9 w-9 items-center justify-center overflow-hidden rounded-xl bg-sidebar-accent/60 text-xs font-semibold text-sidebar-foreground/80 outline-2 outline-offset-2 outline-primary/0 transition-[outline-color]",
-                  isActive ? "outline-primary" : "hover:outline-primary/50",
+                  !isActive && "hover:outline-primary/50",
                 )}
               >
                 {iconUrl ? (

desktop/tests/e2e/community-rail.spec.ts

@@ -66,13 +66,21 @@ test.describe("community rail", () => {
     await expect(buttonA).toBeVisible();
     await expect(buttonB).toBeVisible();
 
-    // The active community is marked via aria-current.
+    // The active community is marked semantically and with a persistent rail.
     await expect(buttonA).toHaveAttribute("aria-current", "true");
     await expect(buttonB).not.toHaveAttribute("aria-current", "true");
-    await expect(buttonA.locator(":scope > span").first()).toHaveCSS(
-      "opacity",
-      "1",
+    const activeIndicator = page.getByTestId(
+      `community-rail-active-${COMMUNITY_A.id}`,
     );
+    await expect(activeIndicator).toBeVisible();
+    await expect(
+      page.getByTestId(`community-rail-active-${COMMUNITY_B.id}`),
+    ).toHaveCount(0);
+    await expect(activeIndicator).toHaveCSS("height", "20px");
+    await expect(activeIndicator).toHaveCSS("width", "4px");
+    await expect(
+      buttonA.locator(":scope > span:not([data-testid])").first(),
+    ).toHaveCSS("opacity", "1");
     await expect(buttonB.locator(":scope > span").first()).toHaveCSS(
       "opacity",
       "1",
@@ -80,7 +88,7 @@ test.describe("community rail", () => {
     const [activeStyle, inactiveStyle] = await Promise.all(
       [buttonA, buttonB].map((button) =>
         button
-          .locator(":scope > span")
+          .locator(":scope > span:not([data-testid])")
           .first()
           .evaluate((element) => {
             const style = getComputedStyle(element);
@@ -88,6 +96,7 @@ test.describe("community rail", () => {
               backgroundColor: style.backgroundColor,
               borderRadius: style.borderRadius,
               color: style.color,
+              outlineColor: style.outlineColor,
               outlineStyle: style.outlineStyle,
               outlineWidth: style.outlineWidth,
             };
@@ -98,6 +107,7 @@ test.describe("community rail", () => {
     expect(activeStyle.borderRadius).toBe(inactiveStyle.borderRadius);
     expect(activeStyle.borderRadius).toBe("12px");
     expect(activeStyle.color).toBe(inactiveStyle.color);
+    expect(activeStyle.outlineColor).toBe(inactiveStyle.outlineColor);
     expect(activeStyle.outlineStyle).toBe("solid");
     expect(activeStyle.outlineWidth).toBe("2px");
     expect(inactiveStyle.outlineStyle).toBe("solid");