fix(desktop): keep startup alive until renderer handoff

[EF1874]

Summary

• Keep the desktop process alive while the gateway is still starting, so a slow browser capability probe cannot make the app exit silently when the splash closes.
• Keep the startup splash lifecycle aligned with renderer handoff.
• Add startup lifecycle and splash regression coverage.
• Include the existing modal interaction and sidebar alignment fixes carried by this branch.

Validation

• npm --prefix App/shell/desktop exec vitest run tests/startup-lifecycle.test.ts tests/startup-splash.test.ts — 2 files, 26 tests passed.
• npm run test -w @memmy/frontend-desktop -- src/components/tests/Modal.test.tsx src/theme/tests/style-alignment.test.ts — 2 files, 23 tests passed.
• git diff --check passed.
• Branch was merged with upstream/v1.1.4 without conflicts.

Closes #362