feat: add left sidebar keyboard shortcut

[salmonumbrella]

Summary

Adds a configurable Command+Backslash shortcut to expand or collapse the left inbox sidebar, matching the existing Command+Slash right-sidebar shortcut pattern.

Changes

• add toggleLeftSidebar to the existing shortcut map with ctrl+\\ as the cross-platform default
• dispatch the action through the shared shortcut bus to the persisted desktop sidebar toggle
• reuse one 0.2s ease transition helper for left-sidebar width and GTD/right-sidebar width and transform animations
• add localized shortcut labels and focused regression coverage

Testing

• npm run lint (frontend and backend)
• npm run build (frontend)
• npm test (frontend: 1,548 passed)
• backend test suite on Node 22 (987 passed)
• npm audit --omit=dev --audit-level=high (frontend and backend)
• git diff --check

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