feat(alerts): rework comments and alert detail panel layout

[kj187]

Summary

Work-in-progress rework of the alert detail panel, grown from a comments-only feature into a broader layout pass. Do not merge — more changes planned on this branch.

• Comments: server-side pagination (5/page), Markdown rendering (bold/italic/strikethrough/inline code/fenced code blocks with syntax highlighting/links/lists), lazy-loaded renderer so the main bundle doesn't grow, taller Write/Preview editor, exact-time tooltip on hover, "jump to latest" affordance when new comments land while paged back.
• Detail panel navigation: replaced the single long scrolling section list with a Details / History / AI Prompt / Comments tab bar (folder-tab style), unified header background.
• Silence banners: collapsed to a slim colored-stripe summary line (action buttons always reachable) instead of a full-color detail box.

Test plan

• Backend: go test -race ./... green
• Frontend: pnpm build, pnpm lint (0 errors), pnpm test:unit:coverage (100% on alertUtils.ts), pnpm duplication clean
• Functional E2E (make e2e-mode MODE=none|internal|oidc) green across all three auth modes
• Pre-commit hooks (Go tests, golangci-lint, pnpm audit/eslint/vitest/jscpd, gitleaks) passed

[codecov]

Codecov Report

❌ Patch coverage is 77.77778% with 6 lines in your changes missing coverage. Please review.
✅ Project coverage is 68.63%. Comparing base (e7464f5) to head (54c1d72).

Files with missing lines	Patch %	Lines
backend/internal/history/store.go	68.42%	4 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #97      +/-   ##
==========================================
+ Coverage   68.51%   68.63%   +0.11%     
==========================================
  Files          39       39              
  Lines        3783     3797      +14     
==========================================
+ Hits         2592     2606      +14     
+ Misses        959      956       -3     
- Partials      232      235       +3     

Flag	Coverage Δ
backend	68.63% <77.77%> (+0.11%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

☔ View full report in Codecov by Harness.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.