[fix] Fix viewport sync in minimap and subgraphs navigation

[christian-byrne]

Fix viewport persistence and minimap syncing:

• Don't use singular 'root' key when persisting viewport, as each workflow tab has its own 'root' so there will be collisions
• The minimap viewport indicator was not updating when graph bounds changed after the canvas was fully loaded. This happened because calculateGraphBounds() would run with new bounds but the viewport position wouldn't recalculate with those new bounds, causing the white viewport box to remain at its initial position until user interaction triggered an update. The fix ensures that when bounds change in updateMinimap(), it also sets the viewport update flag to trigger updateViewport(), keeping the viewport indicator synchronized with the actual graph bounds. This also removes the workaround watchers added in [fix] Make minimap reactive to subgraph context changes #4597 which were patching the symptom but are now unnecessary since the core issue is resolved.

Fixes #4605.

┆Issue is synchronized with this Notion page by Unito

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