Refine canvas workflows and consolidate file previews

[PaulHorn]

Summary

• consolidate standalone document panels into editor-backed file previews and migrate saved document panel state
• add a full-screen recent screenshot viewer with navigation, zoom, download, and drawing annotations
• refine canvas grouping and visibility, Files navigation, docking, browser controls, repository views, and appearance behavior

Validation

• npm run build passes
• npm test completed with 3,713 tests passing and one unrelated filesystem-watch timeout; rerunning src/main/ipc/fsWatch.test.ts passes both tests
• ESLint on changed TypeScript files reports 0 errors and 17 warnings
• repository-wide typecheck remains blocked by two existing territoryGL.test.tsx WebGL/GPU context type errors
• repository-wide lint remains blocked by existing errors outside this change set