feat(web): adopt USWDS civic design system, word-level redline diffs, and bump LTS deps

[williamzujkowski]

Summary

This PR modernizes the US Code Tracker website with an authoritative civic design system, statutory reading enhancements, and dependency upgrades.

1. USWDS 3.x & Civic Tech Design System

• Civic Disclosure Banner (CivicBanner.astro): USWDS usa-banner disclosure banner clarifying independence from government agencies while explaining the verifiable House OLRC XML source and transparent Git version control.
• USWDS Color & Typographic Tokens (global.css): Calibrated official USWDS blue-warm-70v (#1A4480), blue-warm-80 (#112F4E), state-blue (#005EA2), mint-50 (#0F6E56), and warm parchment neutrals (#FAF9F6, #F0EDE8) to guarantee WCAG 2.2 AAA readability without eye glare.
• USWDS Summary Box (SummaryBox.astro): Replaces the statute metadata bar with a structured usa-summary-box featuring legal status tags (In Force, Repealed, Transferred), reading time estimates, OLRC deep links, and a 1-click clipboard citation copy button with toast feedback.
• USWDS Side Navigation (BaseLayout.astro): Added active state indicators (border-l-3 border-state-blue), section counts, and polished Public Sans navigation.
• Hero & Homepage (index.astro): 4-pillar architectural cards, tabular metric cards, and recent amendment cards.

2. Legal Reading & Statutory Diff UX

• Word-Level Legal Redline (DiffViewer.svelte): Intraline word-level LCS tokenization that highlights exact statutory phrasing changes with <del class="diff-del">/ <ins class="diff-add">, rather than whole-paragraph line diffs.
• Split Side-by-Side Mode: Toggle between Unified Legal Redline, Side-by-Side (Split), and raw Git Patch.
• Reader Accessibility Toolbar (ReadingToolbar.svelte): Allows readers to toggle Serif vs. Public Sans, scale font size (17px/19px/21px), and toggle reading measure (72ch vs. 92ch) with localStorage persistence.
• "Hanging Rail" Subsection Guide Lines: Eliminates the mobile "pyramid of doom" indentation while providing subtle vertical guide rails on nested paragraphs (a), (1), (A), (i).

3. Dependency Modernization & Security

• Upgraded Astro to 7.3.5 (resolving XSS advisory in Astro < 7.0.4).
• Added override for postcss-selector-parser: 7.1.6 (resolving moderate ReDoS advisory).
• Updated svelte (5.57.1), turbo (2.11.7), fast-xml-parser (5.11.2), sanitize-html (2.18.0).
• Eliminated deprecated Astro z import and deprecated markdown.rehypePlugins options.
• pnpm audit reports 0 known vulnerabilities.
• pnpm test, pnpm run lint, and pnpm run typecheck pass 100%.