feat: direct nav stack creation and auto-derived navigation structure

[trmquang93]

Summary

• Auto-derive navigationStructure from screenGroups via useMemo — 0 nav-stack groups → null, 1 → single-stack, 2+ → tab-bar. Eliminates manual state that could drift out of sync with the canvas.
• Remove ScreensPanel nav config UI — the dropdown and stack checkboxes are gone; structure is always derived from canvas groups.
• BatchSelectionBar "Stack" button — rubber-band select screens then click Stack to group them into a nav stack with the first screen auto-set as entry.
• Right-click context menu "Create new nav stack…" — creates a nav stack containing the clicked screen, with that screen set as stackEntryScreenId.
• Persistence cleanup — setNavigationStructure removed from useImportExport; on import the value is re-derived from the imported screenGroups.

Test plan

• Rubber-band select 2+ screens → click "Stack" in bar → nav stack group appears on canvas; navigationStructure.type becomes single-stack
• Add a second nav stack group (right-click → "Create new nav stack…") → type becomes tab-bar
• Delete a nav stack group → type reverts correctly
• Toggle a group type from AREA to STACK in ScreenGroup header → navigationStructure updates automatically
• Export → re-import → nav structure re-derives correctly from imported screenGroups
• Generate instructions → stacks appear in navigation.md and build-guide.md
• ScreensPanel no longer shows navigation dropdown or stack checkboxes
• npm test — all 229 tests pass

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Project	Deployment	Actions	Updated (UTC)
flow-forge	Ready	Preview, Comment	Mar 16, 2026 3:07am