UI improvements: diff rendering + 20 Twig templates + asset bundle

[sjelfull]

• Add DiffRenderer service wrapping jfcherng/php-diff + caxy/php-htmldiff
• Register diffRenderer component + expose via craft.audit.diff in Twig
• AuditDiffAsset bundle + diff.css (namespaced, dark-mode via CSS custom
  properties and both media-query + body.dark selectors)
• 20 per-handler Twig templates under src/templates/_diff/:
  index (dispatcher), plain, color, richtext, date, boolean, option,
  multi-option, relation, money, table, matrix, neo, supertable, vizy,
  seo, generic, config, error, truncated
• Register asset bundle on audit/index and audit/details actions
• 14 DiffRendererTest cases (truncation, identical, empty, HTML/text/JSON,
  XSS escape, malformed HTML fallback)
• Tests: 114 passed, 1 skipped (was 100 passed) — +14

[sjelfull]

This change is part of the following stack:

• Phase 1.1 — Foundation: AuditEvent enum + JSON snapshot migration #90
  • Core services: AuditRecorder + FieldDiffService + HandlerRegistry #100
    • Field handlers: 15 handlers for native + third-party fields #101
      • Project config tracking: declarative event handlers for 8 config areas #102
        • UI improvements: diff rendering + 20 Twig templates + asset bundle #103 ◀
          • Tests: add migration + integration + enum coverage (+19 tests) #104
            • Decompose AuditService: extract ElementHandler, UserHandler, UserGroupHandler #105
              • Decompose AuditService: extract SchemaHandler, RouteHandler, BackupHandler, PluginHandler, SettingsHandler #106
                • Repoint event wiring from AuditService to handler services #107
                  • Migrate Pest Feature tests from AuditModel::EVENT_* constants to AuditEvent enum #108
                  • CI: add PHP 8.2/8.3/8.4 matrix, trigger on v3 branch, bump code-analysis to PHP 8.3 #109
                  • Phase 3.0 — Docs: ship escape hatch (filter-records + getting-started + events-and-recording) #110

_{Change managed by git-spice.}

[sjelfull]

Closing — duplicate created by gs stack submit when its tracking metadata fell out of sync. The original PR has been force-updated with the corrected SHA. See #93.