RC2 replacement package preview

Adds a DJI RC2 replacement-package export alongside the normal KMZ export.
Reproduces the externally visible DJI Fly slot structure from the supplied RC2 backup: waypoint/<UUID>/<UUID>.kmz, waypoint/<UUID>/image/ShotSnap.json, and waypoint/map_preview/<UUID>/<UUID>.jpg.
Generates a 1920×1080 mission preview from the Planner map; if the live map canvas cannot be serialized, SkyMission creates a route-only fallback preview instead.
Requires the UUID of an existing DJI Fly dummy mission slot because DJI Fly does not register an arbitrary newly copied waypoint folder as a new mission.
Saves and verifies the current Planner state before creating the RC2 package, and carries the existing low-battery export confirmation into the package workflow.
Adds an in-Planner RC2 export dialog with explicit instructions to extract over Android/data/dji.go.v5/files/ and restart DJI Fly.