Add admin dashboard pages and TOTP settings

[TheSilkky]

Summary

• split the private admin dashboard into overview, accounts, incidents, and settings pages with responsive navigation
• add explicit settings configuration groups for Auth, Storage, Registration, Relay, and Rate limits
• add admin settings TOTP enrollment with QR/manual setup details shown only during active setup, plus confirmation and session verification support
• keep TOTP first-class when WebAuthn is unavailable and leave email challenge as the lower-assurance fallback

Validation

• gofmt -w ./cmd ./internal ./migrations
• go test ./...
• go vet ./...
• scripts/check-markdown-links.py
• git diff --check

Local review

• 20-code-review.md: no blockers
• 30-security-review.md: no blockers
• 50-mdn-web-security-header-review.md: no blockers
• 40-documentation-update.md: skipped; no docs changed
• 76-request-codex-pr-review.md Option B: no blockers