fix(budget): print polish — page margins, title spacing, nested group boxes (#1325)

[steilerDev]

Summary

• Per-page top/bottom margins via @page { margin: 1.5cm 1cm } (works on continuation pages; replaces first-page-only body { padding } approach)
• Reduced top spacing before page h1 in print via PageLayout @media print overrides
• Nested grayscale visual group boxes: outer medium-gray rectangle around each area + all descendants; inner light-gray rectangle around each item + its budget lines. Uses :has() selectors for bottom edges and box-shadow: inset for inner edges.

Fixes #1325

Test plan

• Unit tests pass (95%+ coverage)
• Integration tests pass
• Pre-commit hook quality gates pass

Co-Authored-By: Claude dev-team-lead (Sonnet 4.6) noreply@anthropic.com
Co-Authored-By: Claude frontend-developer (Haiku 4.5) noreply@anthropic.com

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