fix: enable rubber-band hotspot draw on instance screens

[trmquang93]

Summary

• The image-area mousedown handler in useHotspotInteraction.js short-circuited on instance screens because raw instance state has no imageData/imageHeight — those fields only exist on the canonical and are merged in at render time by resolveInstanceVisuals. Resolved at the call site so the guard sees the inherited image without leaking render-time semantics into write paths.
• Added test coverage for remaining instance-editing affordances (select / resize / connect-handle drag / alt-shortcut) in a new useHotspotInteraction.test.js.
• Added a round-trip persistence guard for instance.hotspots[] through importFlow (canonical+instance pair preservation, stringify→import fixed point, orphan demotion preserves locals).

Completes the success criteria from .plans/instance-hotspots-and-connections-plan.md that were missing post-#47.

Test plan

• npx vitest run src/hooks/useHotspotInteraction.test.js src/utils/importFlow.test.js — 43 passed
• npx vitest run — full suite 47 files / 846 tests, all green
• Manual: open a flow with a canonical+instance, drag to draw a hotspot on the instance — confirms a new local hotspot appears only on that placement
• Manual: save and reload — confirm the instance hotspot persists