fix(web): persist diff view mode

[leorivastech]

What Changed

• Move the Diff panel's stacked / split render mode from component-local state into the persisted useDiffPanelStore.
• Include the render mode in the store's existing persisted-state projection.
• Add a regression test covering storage and rehydration.

Why

Closing the right panel unmounts DiffPanel, so its local state always returned to stacked when the panel was opened again. Keeping the preference in the already-persisted Diff panel store preserves it across panel close/reopen and app reloads.

Closes #5497.

UI Changes

Before: selecting the side-by-side view and closing the right panel reset the Diff to unified view.

After: the selected side-by-side view remains active after closing and reopening the right panel and Diff surface.

Testing

• vp test apps/web/src/diffPanelStore.test.ts --run
• tsgo --noEmit -p apps/web/tsconfig.json
• vp lint apps/web/src/components/DiffPanel.tsx apps/web/src/diffPanelStore.ts apps/web/src/diffPanelStore.test.ts
• vp fmt --check apps/web/src/components/DiffPanel.tsx apps/web/src/diffPanelStore.ts apps/web/src/diffPanelStore.test.ts
• Integrated browser verification using an isolated T3 state directory and the real branch diff

Checklist

• This PR is small and focused
• I explained what changed and why
• I included before/after evidence for the UI behavior change
• I included a video for the interaction change

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Note

Low Risk
UI-only preference wiring with existing persisted store; no auth, data, or diff-fetch behavior changes.

Overview
Fixes the Diff panel resetting to stacked (unified) view whenever the right panel closes, by moving the stacked / split preference out of DiffPanel local state and into the existing persisted useDiffPanelStore.

diffRenderMode is stored with the same localStorage persistence as other diff-panel state (partialize), so the choice survives panel unmount/remount and reloads. A store test covers partialization, storage, and rehydration.

^{Reviewed by Cursor Bugbot for commit 4baa37f. Bugbot is set up for automated code reviews on this repo. Configure here.}

Note

Persist diff view mode across sessions in DiffPanel

Moves diffRenderMode from component-local state in DiffPanel.tsx into the Zustand store diffPanelStore.ts, where it is included in the persisted partition. The selected mode ("stacked" or "split") now survives page reloads.

^{Macroscope summarized 4baa37f.}

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[macroscopeapp]

Approvability

Verdict: Approved 4baa37f

This is a simple UI preference persistence change that adds the diff render mode (stacked/split) to the existing zustand store's persistence. The change follows established patterns, includes test coverage, and has no runtime behavior impact beyond remembering user preference.

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