checks: trash icon on the open check bails back to project view

[tyxla]

Summary

Addresses @oandregal's follow-up on #196.

• The trash icon next to a check in the right sidebar previously deleted the file on disk but left the editor showing a stale, now-deleted document — unlike the three-dot menu's Delete, which bails back to the project view.
• When the deleted check is the one open in the middle, DraftEditorScreen now calls onBack() after a successful delete (matching the three-dot menu).
• Before navigating back, the editor pins the sidebar to the Checks tab via onSidebarOpenChange(true) / onSidebarTabChange('checks') so the panel stays open across the transition. (Sidebar open/tab state already lives in App.tsx since Persist right-panel state across project ↔ editor navigation #212, so the transition is smooth without touching uiPrefs directly.)
• useProjectChecks.handleDeleteCheck now returns the IPC result so the editor can react to success / failure.
• Deleting an unrelated check (one that isn't open in the middle) is unchanged — the editor stays put and only the targeted row is removed.

Test plan

• Open a check in the middle, click the trash icon on its row, confirm — editor closes, project view appears with the Checks panel still open, file is gone from disk.
• Open a check, click the trash icon on a different check in the sidebar, confirm — editor stays open on its current file, only the targeted check is removed.
• From project view (no editor open), click the trash icon on a check, confirm — file is deleted and the sidebar stays open on Checks.
• Regression: the three-dot menu's Delete in the editor titlebar still works as before.
• `npm run test:e2e -- tests/e2e/project-checks.spec.ts` (note: the local e2e harness on my machine is currently unable to open the packaged-app window — fails on `trunk` too — so this needs to be validated in CI).

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