Fix day-of-year, compass point, async context and display defects; add tests

[Gabriel-Dalton]

A few real defects found while reading through the app, plus tests covering them.

Logic

• getDayOfYear returned 0 for January 1st and could shift by a day across a DST boundary because it subtracted local DateTimes. Day-of-year feeds foliar moisture content, so this changed predictions.
• degreesToCompassPoint was off by half a sector. 20 degrees reported "N" instead of "NNE". It could also throw on values outside 0-360 instead of wrapping.
• Loaded settings were not pinned to their valid ranges. A stale saved FFMC, curing, wind azimuth or aspect could restore the app into a state it could not calculate from.

Crash risks

• Two BuildContext uses across await gaps (the disclaimer dialog and the delayed scroll) with no mounted check.
• _getPosition returns null on error, but the caller dereferenced the result.

Display

• Fire size rendered the literal string "null" when unavailable, and the label read "60.0 minute fire size".
• Wind direction and aspect rendered raw doubles inconsistently ("157.5" next to "0.0").
• The FMC screen showed unrounded floats with no units.
• Sentence-case labels, a tooltip on the previously unlabelled location button, a dead switch default and a duplicate dev dependency in pubspec.yaml.

Verification

flutter analyze reports no errors (remaining items are the pre-existing CFFDRS naming lints). flutter test passes 25/25, including 10 new tests in test/fire_test.dart covering day-of-year, compass points, intensity classes and range pinning.

Copy and grammar fixes are kept in a separate PR.