feat(desktop): lead onboarding with Nous Portal + fix fresh-install detection (#34970)

- Feature Nous Portal as the primary onboarding card (Recommended tag,
  app logo, single pitch line); collapse other OAuth providers behind an
  "Other providers" disclosure whose open/closed state persists.
- Surface OpenRouter as a one-click API-key option inside the disclosure;
  move "I have an API key" to a quiet bottom-right link.
- Treat "no provider configured" as a normal onboarding state, not a red
  error banner (provider-setup-errors copy match).
- Fix setup.runtime_check: it reported ready when the resolved runtime had
  an empty credential or only implicit Bedrock/IAM, so fresh installs never
  saw onboarding. Now requires a usable credential.
- Auto-wire Windows fonts for WSL2 users so the renderer renders real
  Segoe UI instead of the DejaVu fallback; make WSL detection env-independent
  via the /proc kernel marker.

apps/desktop/electron/bootstrap-platform.test.cjs

@@ -8,7 +8,8 @@ const { bundledRuntimeImportCheck, isWindowsBinaryPathInWsl, isWslEnvironment }
 test('isWslEnvironment detects WSL2 env vars on linux', () => {
   assert.equal(isWslEnvironment({ WSL_DISTRO_NAME: 'Ubuntu' }, 'linux'), true)
   assert.equal(isWslEnvironment({ WSL_INTEROP: '/run/WSL/123_interop' }, 'linux'), true)
-  assert.equal(isWslEnvironment({}, 'linux'), false)
+  assert.equal(isWslEnvironment({}, 'linux', '6.6.87.2-microsoft-standard-WSL2'), true)
+  assert.equal(isWslEnvironment({}, 'linux', '6.6.87-generic'), false)
   assert.equal(isWslEnvironment({ WSL_DISTRO_NAME: 'Ubuntu' }, 'darwin'), false)
 })