docs(04): capture phase context

Phase 04: Drag Interaction
- Implementation decisions documented
- Phase boundary established

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

.planning/phases/04-drag-interaction/04-CONTEXT.md

@@ -0,0 +1,61 @@
+# Phase 4: Drag Interaction - Context
+
+**Gathered:** 2026-01-18
+**Status:** Ready for planning
+
+<domain>
+## Phase Boundary
+
+User can drag itinerary items with proper visual feedback, animation, and constraint enforcement. Items include travel segments and custom items (games are not draggable). The system uses semantic positions (day, sortOrder) established in Phases 1-3.
+
+</domain>
+
+<decisions>
+## Implementation Decisions
+
+### Drag Feel & Weight
+- Quick & snappy lift — instant pop-up, light and responsive (iOS Reminders style)
+- 1:1 finger tracking — item stays exactly under finger, no lag or smoothing
+- Immediate shuffle — items move instantly when drag crosses them (no delay threshold)
+- Subtle scale on lift (1.02-1.03x) — barely noticeable, just enough shadow change
+
+### Insertion Line Design
+- Color follows selected theme in Settings — respects user's theme choice
+- Fade in/out animation (~150ms opacity transition)
+- Plain line only — no glow, shadow, or endpoint embellishments
+
+### Invalid Drop Feedback
+- Red tint on dragged item while hovering over invalid zone
+- Quick spring snap-back animation (~200ms with slight overshoot)
+- Error haptic pattern (3 quick taps) — distinct "nope" feeling
+- No sound effects — haptic and visual feedback only
+
+### Claude's Discretion
+- Insertion line thickness (suggested 2-4pt)
+- Shadow depth and exact tilt angle during drag
+- Drop settle animation timing
+- Auto-scroll speed and dead zone sizing (user didn't discuss this area)
+- Line positioning relative to items
+
+</decisions>
+
+<specifics>
+## Specific Ideas
+
+- Lift feel should match iOS Reminders — quick, light, responsive
+- Snap-back should have slight overshoot for spring physics feel
+- Theme-aware colors for insertion line
+
+</specifics>
+
+<deferred>
+## Deferred Ideas
+
+None — discussion stayed within phase scope
+
+</deferred>
+
+---
+
+*Phase: 04-drag-interaction*
+*Context gathered: 2026-01-18*