feat: add per-tab theme override groundwork

[oz-for-oss]

Closes #478

Summary

• Adds the per-tab theme override data model (TabThemeState) with menu, launch-config, cwd, and window-default slots.
• Adds launch-configuration theme: fields for tabs/windows and save logic for preserved overrides.
• Adds built-in theme reference resolution, local-only directory override settings/matching helpers, feature-flag wiring, and persistence through session restore.

Validation

• cargo fmt --manifest-path /workspace/warp/app/Cargo.toml
• cargo check --manifest-path /workspace/warp/Cargo.toml -p warp --lib
• git -C /workspace/warp --no-pager diff --check

Filtered unit-test builds were attempted, but the sandbox killed rustc with SIGKILL during lib-test compilation even with CARGO_BUILD_JOBS=1.

Notes

This is a scoped implementation update for the foundation of per-tab theme overrides. Remaining follow-up work includes renderer catalog/call-site migration, menu actions, and live directory/settings recomputation before the feature is fully user-visible.

Conversation: https://app.warp.dev/conversation/0f295f64-0c19-4e7f-a295-52d65342f327
Run: https://oz.warp.dev/runs/019def12-b234-75a7-95d7-9c3661c0c6de
Plans:

• Implementation plan for GH478 scoped per-tab theme groundwork
  This PR was generated with Oz.

[captainsafia]

@gulsahsarsilmaz Looks like you've already seen this based on your reaction above but you're welcome to test this out locally/steer Oz on this PR to make changes as needed.

[zachlloyd]

@captainsafia one issue here is that oz didn't implement the whole thing. We need a strategy that supports multiple PRs per issue I think.

@oz-agent can you do the complete e2e implementation please

[oz-for-oss]

@zachlloyd

I'm working on changes requested in this PR (responding to a PR conversation comment). Spec context was found and will be used to ground the change.

You can view the conversation on Warp.

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