Allow typing an exact camera direction

[bbruce1]

TLDR

The direction on a node could only be set by dragging the slider, which makes an exact degree hard to get on small screens. Render the current direction in the label as a small numeric field so it can be typed directly. Whole numbers from 0 to 359 are applied then anything else is discarded and the field resyncs.

Fixes #64

What changed

• The current direction in the "Directions:" label is now an editable numeric field (a WidgetSpan instead of a bold TextSpan), in both the add and edit node sheets.
• Entering 0-359 applies it and anything else is discarded and the field resyncs. Field and slider stay in sync both ways..

Testing

1. Tap New Node, pick a profile that needs a direction.
2. Tap the number in the "Directions:" label, type 270, press Return slider and map cone follow. Drag the slider and the field follows back.
3. Type 999 or abc, press Return and the field snaps back and nothing moves.
4. Repeat on an existing camera via Edit*

flutter analyze clean; flutter test passes.