Simplify resizable API: use persist: false | 'localStorage' | fn

[Copilot]

Consolidates resizable prop configuration from separate {persist: false} and {save: fn} patterns into unified {persist: false | 'localStorage' | fn} API.

Changelog

Changed

• resizable prop now accepts {persist: false | 'localStorage' | fn} instead of separate {persist: false} and {save: fn} configs
• Renamed CustomPersistConfig → PersistConfig
• Renamed NoPersistConfig → removed (merged into PersistConfig)
• Custom persistence functions now passed via persist property instead of save

Before:

<PageLayout.Pane resizable={{save: (w) => store(w)}} />

After:

<PageLayout.Pane resizable={{persist: (w) => store(w)}} />
// Explicit localStorage now possible:
<PageLayout.Pane resizable={{persist: 'localStorage'}} />

New

• PersistFunction type for custom persistence
• isCustomPersistFunction type guard
• isPersistConfig type guard

Removed

• NoPersistConfig type (merged into PersistConfig)
• CustomPersistConfig type (replaced by PersistConfig)
• isNoPersistConfig type guard
• isCustomPersistConfig type guard

Rollout strategy

• Patch release
• Minor release
• Major release; if selected, include a written rollout or migration plan
• None; if selected, include a brief description as to why

Note: While this changes the API surface, the original PR hasn't been released yet, so this is effectively a pre-release refinement.

Testing & Reviewing

• Updated 66 unit tests to use new API
• Updated Storybook examples demonstrating all three persist modes
• Type checking passes with new signatures

Merge checklist

• Added/updated tests
• Added/updated documentation
• Added/updated previews (Storybook)
• Changes are SSR compatible
• Tested in Chrome
• Tested in Firefox
• Tested in Safari
• Tested in Edge
• (GitHub staff only) Integration tests pass at github/github (Learn more about how to run integration tests)

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Latest commit: 70cbcfd

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[github-actions]

👋 Hi, this pull request contains changes to the source code that github/github-ui depends on. If you are GitHub staff, test these changes with github/github-ui using the integration workflow. Or, apply the integration-tests: skipped manually label to skip these checks.