feat(domain): support 6/8/10-char Maidenhead grid square conversion

Port the 8-char (4-pair) Maidenhead grid algorithm from the
"QTH定位器 2.0" app (com.us1pm.gridsquarelocator) into QthConverter
so locator precision matches common grid tools instead of being
truncated to 6 chars.

Previously positionToQth() emitted only 6-char locators and
qthToPosition() discarded everything past the 6th character via
take(6), losing the finer 30" x 15" resolution carried by 8-char
grid squares.

What changed:
- positionToQth(lat, lon, precision = 8) now emits 8-char locators
  by default; precision = 6 / 10 available for backwards
  compatibility and maximum resolution (1.25" x 0.625").
- qthToPosition() parses 6/8/10-char locators and returns the center
  of the finest encoded cell (30" x 15" for 8-char, 1.25" x 0.625"
  for 10-char) instead of the 6-char cell center.
- Locator validation regex tightened: 6/8/10 chars accepted,
  4-char strings like "JN58" are now rejected as invalid.
- Boundary clamping added so lat = 90 / lon = 180 no longer overflow
  the A-R / 0-9 / a-x alphabet (previously produced invalid chars).

Also fixed a pre-existing compile error in RadarView.kt: a delegated
property was assigned after declaration ("by" on an already declared
val). Converted the sweep angle to an if/else expression.

Verification:
- QthConverterTest extended to 5 test cases covering 6/8/10-char
  roundtrips, invalid input, boundary coordinates and roundtrip
  stability.
- Cross-checked against a Python reference model of the decompiled
  APK algorithm: 20k random roundtrips at 8 and 10 chars, 0 failures.
- :core:domain:test green; :app:compileDebugKotlin passes.

feature/radar/src/main/java/com/rtbishop/look4sat/feature/radar/RadarView.kt

@@ -97,17 +97,17 @@ fun RadarViewCompose(
     // Drive the sweep from the animation framework to eliminate state mutation inside the draw block.
     // Only create the infinite transition when the sweep is enabled — otherwise the canvas would
     // be invalidated every frame for an effect that isn't visible, wasting GPU and battery.
-    val sweepDegrees: Float
-    if (shouldShowSweep) {
+    val sweepDegrees = if (shouldShowSweep) {
         val sweepTransition = rememberInfiniteTransition(label = "sweep")
-        sweepDegrees by sweepTransition.animateFloat(
+        val animatedSweep by sweepTransition.animateFloat(
             initialValue = 0f,
             targetValue = 360f,
             animationSpec = infiniteRepeatable(tween(SWEEP_DURATION_MS, easing = LinearEasing)),
             label = "sweepDegrees"
         )
+        animatedSweep
     } else {
-        sweepDegrees = 0f
+        0f
     }
     val measurer = rememberTextMeasurer()
     val sunPainter = painterResource(R.drawable.ic_sun)