fix(desktop): hide resting markdown link underlines

[camaragon]

Summary

• remove the always-on underline from external Markdown links
• retain an underline on hover for pointer users
• prevent the blue task-ID underline seen in the mobile timeline

Verification

• node --import ./test-loader.mjs --experimental-strip-types --test src/shared/ui/markdown/ExternalLinkAnchor.test.mjs
• pnpm exec biome check src/shared/ui/markdown/ExternalLinkAnchor.tsx src/shared/ui/markdown/ExternalLinkAnchor.test.mjs
• pnpm typecheck
• pnpm build

[github-actions]

🔐 Codex Security Review

Status: review required for the current range.

The current range is b49675894b39f87215e4dfc8c1ad4a3c28c6097e...5c65bfb8d22545eff6b9197c68a90eb4ff67d7b9.
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