Add polished Artifacts explorer gallery

[ashwin-pc]

Summary

• add a distinct Workspace / Artifacts explorer with responsive gallery cards, folder navigation, lazy thumbnails, and rich image, HTML, Markdown, video, PDF, and text previews
• preserve independent workspace and artifact navigation state, including folders, scroll positions, editors, previews, and sandboxed iframe state
• integrate file and artifact navigation with browser history so Back returns previews to their tree/gallery before closing the panel
• add session-scoped artifact media lookup, MIME handling, and byte-range responses for seekable videos

Tests

• npm test
• added desktop/mobile E2E coverage for browser Back and Forward through file tree, artifact gallery, and artifact preview states
• added artifact gallery/preview visual snapshots
• added API coverage for artifact MIME types and byte-range responses

[chatgpt-codex-connector]

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