feat: shared screens / reusable components

[trmquang93]

Summary

• Let users explicitly mark screens as reusable components so AI build instructions describe each component once and link from every placement, instead of regenerating the same code multiple times.
• Instances mirror their canonical's image and hotspots at render time — edits on the canonical propagate live; hotspot editing is canonical-only.
• Generated instructions add a components/<slug>.md file per canonical with a Placements table; instance entries in screens.md are stubs that link to it.

Implementation

• Data model (useScreenManager.js): adds componentId / componentRole to screens, new setScreenComponent(id, mode) action with auto-promotion of the first instance when a canonical is deleted; duplicateSelection demotes copied canonicals to instances.
• Backward compat (importFlow.js): backfills missing fields and enforces one canonical per componentId (drops orphan instances, demotes duplicate canonicals).
• UI: Sidebar Component section (mark canonical / "Instance of…" picker / unlink / open canonical), ScreensPanel context-menu items, ScreenNode purple border + ⟳ Component / ↗ Instance pills, theme tokens.
• Render-time visual swap (resolveInstanceVisuals.js): pure helper that merges canonical's imageData, dimensions, and hotspots into an instance at draw time. Hotspot edits, + Link, drag-to-create, and resize handles are gated off on instances.
• Instruction generation (generateInstructionFiles.js): exported getComponentGroups; instances render as a stub linking to the canonical's component file; canonicals get a "see component file" header; main.md adds a components/ row only when canonicals exist; new generateComponentFiles emits per-component markdown with a Placements table (canonical + instances, link counts) and full spec.
• MCP (mcp-server): update_screen accepts componentId (string|null) and componentRole (canonical|instance|null).
• Docs: user guide gets a "Reusable Components" section.

Test plan

• npm test — 685 tests across 36 files passing (added 8 for resolveInstanceVisuals + 10 for component grouping = 18 new)
• npm run lint — 0 errors, 0 warnings
• npm run build — success (only pre-existing chunk-size advisory)
• Manual: mark canonical → mark another instance → instance image swaps to canonical's, hotspots mirror, edits propagate live
• Manual: delete canonical → first instance auto-promotes
• Manual: duplicate a canonical → copy is an instance, not a second canonical
• Manual: generate instructions → exactly one components/<slug>.md per canonical, instance entries are stubs