Add PlanTree hybrid tree/node visualizer

[JMRussas]

Summary

• Implements full PlanTree component suite with 19 files: recursive tree rendering, SVG dependency overlay (bezier curves between dependent tasks), CSS grid expand/collapse animations, WAI-ARIA treeview keyboard navigation (roving tabindex, arrow keys, Home/End/Escape), search/filter with <mark> text highlighting, and runtime HSL theme configurator with localStorage persistence
• Adds centralized expand state (useExpandState), keyboard hook (useTreeKeyboard), search hook (useTreeSearch), and reactive theme hook (usePlanTreeTheme with MutationObserver for dark/light detection)
• 52 new frontend tests (211 total), zero external dependencies added

Test plan

• cd frontend && npx tsc --noEmit — clean
• cd frontend && npm test — 211 tests passing (26 test suites)
• Manual: open a project with an L2 plan, verify tree renders with colored nodes, dependency lines, keyboard navigation, search filtering, and theme configurator

Generated by Claude Code · Claude Opus 4.6

[JMRussas]

Review Defect Fixes (commit 4472667)

8 defects found during self-review, all fixed:

High Severity

#	File	Defect	Fix
3	PlanTreeNode.tsx:40	Rules of Hooks violation — useRef, useRegisterNode, useDependencyContext, useCallback called after conditional return null. React would throw when a node transitions visible→invisible.	Moved all hook calls above the early return

Medium Severity

#	File	Defect	Fix
1	useTreeKeyboard.ts:76, index.tsx:51	CSS selector injection — querySelector(\[data-node-id="${id}"]`)vulnerable if ID contains"or]`	Use CSS.escape(id)
6	NodeDetail.tsx:26	Double Escape handler — global keydown listener for Escape conflicts with useTreeKeyboard's Escape handler, calling onClose() twice	Removed global listener; Escape handled by useTreeKeyboard
5	ThemeConfigurator.tsx:59	Stale closure — if (open) setActive(null) captures previous render's open value	Use callback form: setOpen(o => { if (o) setActive(null); return !o })

Low Severity

#	File	Defect	Fix
2	DependencyContext.tsx:60	O(n) downstream scan per node per render (~12,800 iterations for 80 tasks)	Pre-compute downstreamMap (reverse index) via useMemo in provider
7	useExpandState.ts:43	Expand state not re-synced when tree identity changes — new node IDs never added to map	useEffect on defaultMap merges new defaults while preserving user overrides
8	DependencyOverlay.tsx:101	Unnecessary recalculation on hover — paths don't move, only styling changes	Removed hover-triggered recalculate() call

Nit

#	File	Defect	Fix
9	PlanTree.css:381-409	CSS formatting inconsistency — 17 rules compressed into single lines	Expanded to multi-line format matching rest of file