fix(ui): calculator polish — smart input steps, unified tech/MAT lock idiom, tech floor, speed-lock ceiling fix

- pkg/calc: DriveForSpeed treats restMass==0 as a valid ceiling-only
  case (every positive drive solves it), so locking the displayed
  speed of a D=1, W=A=S=C=0 ship is no longer a phantom "infeasible".
- ship-design-area: drive/weapons/shields/cargo inputs use a JS-driven
  smart step on ArrowUp/ArrowDown (0↔1 jump, otherwise ±0.1) and hide
  the native spinner so it cannot produce invalid (0, 1) values;
  armament keeps its native step 1.
- Tech and planet MAT cells follow the same lock idiom as goal-seek
  locks: open padlock (🔓) over the inherited value → click to open
  an input with a closed padlock (🔒). The padlock slot is always
  reserved, so the column width is stable.
- Tech overrides (design area and modernization target) are floored
  at the player's current tech on this turn — a lower value is
  flagged as invalid.

pkg/calc/solve_test.go

@@ -24,12 +24,30 @@ func TestDriveForSpeed(t *testing.T) {
 	if !ok || math.Abs(got-drive) > 1e-9 {
 		t.Errorf("DriveForSpeed round-trip = %v (ok=%v), want %v", got, ok, drive)
 	}
-	// Speed can never reach the stripped-hull ceiling 20*driveTech.
+	// With a positive restMass speed can never reach 20*driveTech.
 	if _, ok := calc.DriveForSpeed(20*driveTech, driveTech, restMass); ok {
 		t.Error("DriveForSpeed at the speed ceiling should be infeasible")
 	}
 }
 
+func TestDriveForSpeedZeroRest(t *testing.T) {
+	// With restMass==0 the only achievable speed is the stripped-hull
+	// ceiling 20*driveTech; any positive drive reaches it. Off-ceiling
+	// targets are infeasible.
+	const driveTech = 1.5
+	ceiling := 20 * driveTech
+	got, ok := calc.DriveForSpeed(ceiling, driveTech, 0)
+	if !ok || got <= 0 {
+		t.Errorf("DriveForSpeed(ceiling, _, 0) = %v (ok=%v), want positive", got, ok)
+	}
+	if _, ok := calc.DriveForSpeed(ceiling/2, driveTech, 0); ok {
+		t.Error("DriveForSpeed(below ceiling, _, 0) should be infeasible")
+	}
+	if _, ok := calc.DriveForSpeed(ceiling+1, driveTech, 0); ok {
+		t.Error("DriveForSpeed(above ceiling, _, 0) should be infeasible")
+	}
+}
+
 func TestShieldsForDefence(t *testing.T) {
 	const shields, shieldsTech, restMass = 5.75, 1.0, 40.0
 	defence := calc.EffectiveDefence(shields, shieldsTech, shields+restMass)