fix(webapp): match the environment path on a segment boundary

The page below the environment was sliced off the front of the pathname on
a bare startsWith, so an environment slug that strictly prefixes another —
branch slugs are `<parent>-<branch>`, making `preview-feat` a prefix of
`preview-feat-2` — yielded a garbled suffix that resolved to no page. The
switcher hooks can hit this mid-navigation, where the pathname comes from
the pending location but the environment path still comes from the current
match.

Co-Authored-By: Claude <noreply@anthropic.com>

Author: claude

apps/webapp/app/utils/pageSwitching.test.ts

@@ -437,6 +437,14 @@ describe("pageBelowEnvironment", () => {
     expect(pageBelowEnvironment("/account/tokens", environmentLocation.pathname)).toBe("");
     expect(pageBelowEnvironment("/orgs/acme/settings/team", environmentLocation.pathname)).toBe("");
   });
+
+  it("gives nothing when the environment path only prefixes the one in the page path", () => {
+    const branch = "/orgs/acme/projects/api/env/preview-feat";
+
+    expect(pageBelowEnvironment(`${branch}-2/runs`, branch)).toBe("");
+    expect(pageBelowEnvironment(`${branch}-2`, branch)).toBe("");
+    expect(pageBelowEnvironment(`${branch}/runs`, branch)).toBe("runs");
+  });
 });
 
 describe("pathForEnvironmentSwitch", () => {

apps/webapp/app/utils/pageSwitching.ts

@@ -101,7 +101,10 @@ export function pageBelowEnvironment(
 ): string {
   if (environmentPathname === undefined || !pathname.startsWith(environmentPathname)) return "";
 
-  return pathname.slice(environmentPathname.length).replace(/^\/+/, "");
+  const below = pathname.slice(environmentPathname.length);
+  if (below !== "" && !below.startsWith("/")) return "";
+
+  return below.replace(/^\/+/, "");
 }
 
 /** The current page in another environment of the same project, keeping filters where they apply. */