Tier 4: Add breakpoint boundary-width tests (767/768, 1023/1024)

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Context

Tier 4 viewport specs test at canonical widths (390, 768, 1280) but don't test the exact CSS breakpoint boundaries. Off-by-one errors in media queries or useBreakpoint() could cause layout glitches at exactly 767px or 1023px.

Scope

Add 1-2 specs that test at boundary widths:

• 767px — should render mobile layout (bottom tab bar, SwipeCard, no sidebar)
• 768px — should render tablet layout (rail sidebar, TxnTable, no bottom tab bar)
• 1023px — should render tablet layout
• 1024px — should render desktop layout (full sidebar)

References

• Breakpoints defined in useBreakpoint.ts: mobile < 768, tablet 768-1023, desktop >= 1024
• Existing tier4 specs at tests/e2e/tier4/ cover canonical widths
• Design spec: docs/superpowers/specs/2026-05-19-tier4-e2e-viewport-design.md