fix(FN-5257): restore SecretsView action icon visibility

- Standardize SecretsView action icon sizing and color so header, row, loading, and modal toggle icons stay visible in dark and light themes
- Expand SecretsView tests to load app CSS and assert icon visibility across both themes
- Stabilize the process supervisor fallback test by removing an unnecessary maxLifetime override

Fusion-Task-Id: FN-5257

packages/core/src/__tests__/process-supervisor.test.ts

@@ -142,7 +142,6 @@ describe("process-supervisor", () => {
 
     const child = superviseSpawn(process.execPath, [fixturePath, "exit-immediately"], {
       stdio: "ignore",
-      maxLifetimeMs: 100,
     });
 
     expect(child.pgid).toBeNull();