fix(web): F14 — close 5-expert audit findings (7 HIGH + 7 MEDIUM/LOW)

[explosivebit]

Summary

5-expert audit (TS / Frontend / Security / Performance / UX) on F11+F12+F13 returned 0 CRITICAL, 7 HIGH, ~12 MEDIUM, ~10 LOW. This PR closes all 7 HIGH + 7 MEDIUM/LOW findings.

HIGH

ID	Finding	Fix
TS-H1	renderBody return type was plain string — caller could feed any string to {@html}	Branded SafeHtml = string & { readonly __safeHtml: unique symbol }
TS-H2	marked.parse(...) as string cast valid only with async: false — silent breakage if option removed	// TODO(marked-types) comment + revisit on marked v19
FE-H1	Matrix impact-mode never faded — CSS selector targeted .node only; Matrix has .row-header / .col-header	Selector extended via :is(.node, .row-header, .col-header)
FE-H2	Notification.permission access throws in sandboxed iframes / restrictive Safari → toggle effect crashed	try/catch in notificationPermission, requestPermission, fire
UX-H1	Long code blocks pushed horizontal scrollbar to entire panel	.artifact-body pre / pre code get max-width: 100%; white-space: pre-wrap; word-break: break-word
PERF-H1	marked + DOMPurify (~21 KB gzip) shipped on first paint despite body collapsed by default	Dynamic import inside bodyExpanded branch — first-paint bundle drops by 21 KB gzip

MEDIUM / LOW

• FE-M2 — clipboard fallback (textarea + execCommand('copy')) for http:// non-secure contexts.
• FE-M3 — liveText prefixed with zero-width-space + liveSeq counter so screen readers re-announce identical breach text.
• FE-M4 — Notify-on first-fire-storm: prime prevHealthSnapshot when user flips notify on mid-session.
• FE-L1 — prefers-reduced-motion query extended from svg * to *, *::before, *::after.
• UX-M1 — tooltips on Show downstream / upstream buttons.
• UX-M3 — .links ul { max-height: 30vh; overflow-y: auto }.
• SEC-L1 — DOMPurify afterSanitizeAttributes hook forces rel="noopener noreferrer" on target="_blank" anchors (CWE-1022 guard).

Backlog (deferred)

• UX-H2 mermaid latent (no current PRD/RFC uses mermaid; ADR for diagram renderer when needed).
• TS-M3..M8, L9..L12 (cosmetic).
• SEC-L2 redaction of secret patterns in body excerpt (needs thoughtful regex set — separate artifact).
• SEC-L3 kindFilter validation (low impact).
• FE-L2..L4 (rare edge cases).

Verify

• npx svelte-check — 0/0/436.
• npm test — 70/70 (68 baseline + 2 new tests for noopener hook + iframe-throw).
• node scripts/smoke.mjs — PASS.
• Playwright DOM check — 0 console errors.

Test plan

• CI smoke matrix (3-OS × Node 22) green.
• svelte-check clean on PR.
• vitest 70/70.
• Manual: open ArtifactPanel for an Epic with many children — link list scrolls within max-height: 30vh.
• Manual: clear all filters in Matrix view with impact-mode active — non-impacted headers visibly fade.
• Manual: open in iframe — Notify toggle does not crash the page.

Refs: PRD-006 RFC-005 PRD-007 RFC-006

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