Improve pull request conversations

[danielss-dev]

Summary

• redesign the hosted pull-request conversation as an avatar-led review timeline
• add a keyboard-operable Markdown composer with Write/Preview modes, formatting actions, character count, and clearer submission feedback
• support hosted screenshot/image Markdown with explicit click-to-load rendering
• show GitHub and Azure DevOps profile avatars with initials as a no-layout-shift fallback
• update the task tracker, roadmap, README, user guide, and durable implementation guidance

Why

The existing conversation view was visually sparse and offered only a plain textarea. Review discussions now have clearer author/context separation and a composer that supports the Markdown workflows people expect without leaving Strand.

User impact

Comments are easier to scan and compose. Provider avatars load lazily with no referrer; arbitrary images in PR content remain inert until the user explicitly reveals them. Direct local binary attachment upload is intentionally not presented because the supported GitHub and Azure CLI paths do not share a stable attachment API.

Validation

• cargo check -p strand-core -p strand-tauri
• cargo test -p strand-tauri (25 passed)
• pnpm --filter ./ui exec vitest run (212 passed)
• pnpm --filter ./ui exec tsc --noEmit