fix: keep selected iOS tab when no workspace is active

[shaw-baobao]

Summary

This PR fixes an iOS tab-state issue where selecting Codex/Git/Log could still end up with Home selected when no workspace was active.

Root Cause

useTabActivationGuard forced phone layouts without an active workspace back to home, overriding user tab selection for codex, git, and log.

Changes

• Removed the phone-specific fallback-to-home logic in useTabActivationGuard.
• Kept the tablet-specific guard unchanged (home/projects -> codex).
• Updated the App.tsx callsite to match the simplified hook API.
• Added regression tests to ensure:
  • phone mode does not force home for non-home tabs without a workspace
  • tablet mode still redirects home to codex

Validation

• npm run lint
• npm run typecheck
• npm run test

[Dimillian]

@codex review

[chatgpt-codex-connector]

Codex Review: Didn't find any major issues. Bravo.

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