feat(viewer): redesign GenericViewer to V8 chrome-less surface

[MrCoder]

Summary

Reimplements GenericViewer.vue as the V8 chrome-less surface from the ZenUML for Confluence design system. Affects all variants (lite, full, diagramly).

• Persistent toolbar replaced with hover-revealed edges:
  • Top edge — title (left, ellipsised, max 420px) + Edit + Fullscreen (right)
  • Bottom edge — centered pill with Copy code · Export PNG · Versions · Copy link
• 3px left product-bar accent (blue / orange / purple via data-app)
• Warm-cream canvas (#F8F7F4) with the system's signature dot pattern
• Resting state shows just "Hover to reveal controls" hint
• Removed: Like button, Send Feedback, Upgrade CTA in viewer header, DEV/STAGING header tint
  • Decision recorded in Flywheel: the gold Upgrade button ran ~1 year with no measurable conversion lift; paywall conversion now relies on the dedicated UpgradePrompt modal triggered by other surfaces. Revisit only as an explicit experiment with success metrics.
• Embedded badge kept as a compact EMBED chip on the top-left edge
• New Copy link action — resolves the parent page URL via requestConfluence(/wiki/api/v2/pages/{id}). Lazy-imported so the standalone preview harness still boots
• Preserves: screen-capture-content class (used by Attachment.ts + ExportModal), wide / hideHeader props, EventBus edit / fullscreen events, showEdit permission gating, all tracking events

Test plan

• pnpm test:unit src/components/Viewer/GenericViewer.spec.ts — 10/10 pass (rewrote spec for the V8 surface)
• ESLint clean on changed files
• pnpm build:lite succeeds
• Local Playwright verification on viewer-preview.html?sandbox=seq-view — frame renders, hover reveals top + bottom edges, Edit click fires EventBus edit, resting hint shows when not hovering, orange Lite product-bar visible, dot-pattern canvas visible
• Forge tunnel verification on a live Confluence site (pending — needs forge auth + dev install)
• Visual review across all three variants (full / lite / diagramly) — product-bar color is variant-driven via data-app

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