fix(themes): sync custom theme selections portably

[nisavid]

Summary

• Store custom theme references under Warp's theme root as relative paths so Settings Sync does not publish machine-specific theme roots.
• Resolve portable references and legacy macOS/Linux absolute theme-root paths against the local theme directory when reading synced settings.
• Allow cloud sync for custom and Base16 theme selections only when every custom theme reference can be represented portably.

Fixes #9723.
Addresses the root cause behind #8988 and #6678.

Relationship to #9634

PR #9634 handles tilde expansion/contraction for home-relative paths. This PR is broader: it stores and resolves custom theme references relative to Warp's platform theme root, so the same selected theme can sync between macOS ~/.warp/themes and Linux XDG data theme roots.

Notes

Settings Sync still syncs only the custom theme reference. It does not upload, download, create, copy, or manage custom theme YAML files.

Test Plan

• cargo test -p warp themes::theme::tests
• cargo test -p warp settings::theme::tests
• cargo fmt --check
• cargo clippy -p warp --lib --tests -- -D warnings
• env PATH=/tmp/codex-corepack-bin:/usr/local/sbin:/usr/local/bin:/usr/bin cargo clippy --workspace --all-targets --tests -- -D warnings
• env PATH=/tmp/codex-corepack-bin:/usr/local/sbin:/usr/local/bin:/usr/bin cargo clippy --workspace --all-targets --all-features --tests -- -D warnings (blocked locally: --all-features enables release_bundle, but warp-channel-config is not installed on PATH)

Co-Authored-By: Oz oz-agent@warp.dev

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[oz-for-oss]

@nisavid

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[oz-for-oss]

Overview

This PR makes custom theme selections portable by serializing theme-root paths relatively, resolving portable and legacy paths against the local theme root, and expanding syncability checks for custom/Base16 selections.

Concerns

• Relative custom theme paths from synced settings are accepted without traversal validation, so .. components can resolve outside the intended theme root.

Security

• The new relative-path resolution should reject or normalize traversal components before applying cloud/settings values to filesystem paths.

Verdict

Found: 0 critical, 1 important, 0 suggestions

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[nisavid]

@cla-bot check

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[nisavid]

/oz-review

[oz-for-oss]

@nisavid

I'm re-reviewing this pull request in response to a review request.

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I reviewed this pull request and requested human review from: @lucieleblanc.

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Oz no longer requests changes for this pull request after the latest automated review.

[oz-for-oss]

Overview

This PR stores custom theme selections as portable theme-root-relative references when possible, resolves those references back to the local theme directory when reading settings, and gates theme setting cloud sync on portable custom theme paths.

Concerns

• No blocking correctness or security concerns found in the changed lines.

Verdict

Found: 0 critical, 0 important, 0 suggestions

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[rrroyal]

i'm pretty sure other warp channels use different paths, i.e. preview uses .warp-preview instead of .warp. does it also handle that case?

windows also uses different paths. maybe it would be better if it used platform-specific logic with conditional compilation instead of "hardcoding" paths here?