Add workspace file previews

[coder-hhx]

Summary

• Add shared workspace file preview overlays for GUI and gateway web, including HTML, Markdown, PDF, images, media, spreadsheets, and Word document previews.
• Render HTML previews from Blob URLs with script-enabled sandboxing so interactive single-page files can run.
• Add macOS legacy doc/rtf-to-HTML preview conversion with content-aware caching, CSV/TSV file icons, and near-real-time file tree refresh.
• Add GUI-only default-app open and containing-folder actions; gateway web intentionally omits local folder opening.

Verification

• cargo test --manifest-path crates/agent-gui/src-tauri/Cargo.toml workspace_preview
• pnpm -C crates/agent-gui build
• pnpm -C crates/agent-gateway/web build
• go -C crates/agent-gateway build -o bin/liveagent-gateway ./cmd/gateway
• go -C crates/agent-gateway test ./...
• git diff --check