[chriskinal]

Fixes #306, fixes #305.

#306 U-turn in headland ignored

From the dump: the turn was armed 27 m from the headland. The planned path's entry leg (16 m, the current pass itself) put the path's first point 6 m behind the tractor. The trigger fired only within 2 m of that first point, so it never fired; and the "entered headland without triggering" reset was an else if behind the trigger branch, so it could never run. The path stayed on screen while the tractor drove 9 m into the headland.

• Trigger on being on the path. The tractor is on the path when it is within 1.5 m of the nearest segment and heading along it (within the existing 20° alignment tolerance). The turn starts there, wherever it joined. The heading test keeps the exit leg and an omega's crossing legs from counting.
• The planner records where the path leaves the pass (ArcStart, through YouTurnCreationOutput → TurnPathResult → YouTurnWorkingState, mirrored in YouTurnState/YouTurnSnapshot). The 20 m approach alarm measures to it rather than to the leg's first point.
• Arc already behind the tractor: the turn is replanned from the tractor's position (the manual turn's arc, to the same next track) and started; with no room for that, the path is dropped with the too-close sound and "U-turn missed: too close to the headland to turn. Take over manually."
• Entered the headland off the path: dropped with the sound and "U-turn missed: entered the headland off the turn path. Take over manually."

Tests (YouTurnTriggerOnPathTests): on the leg → starts; beside the path, or heading the other way → not; short of the path → countdown published; alarm measures to the arc; in the headland off the path → dropped with sound and message; arc behind → replanned and executing; and through the real planner, a turn armed 15 m before the turn line (path start 7 m behind the tractor, as in the report) starts on the same tick. YouTurnDistanceToTriggerTests paths now run along the AB line so the on-path rule applies.

#305 Delete applied area with no field

Delete Applied Area, Boundary and Field Builder show "Open a field first" instead of acting on nothing; DeleteAppliedAreaConfirmed refuses on the host too.

Checks

• Models 167, ViewModels 308, Services 1512 tests pass; i18n-extract.py --check clean; Desktop builds.
• Not replayed on the bench against the reporter's field; the planner test reproduces the geometry of the report.

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