Fix: ms exact mass (M+)

[Nicolass67]

Summary

• show Exact mass (M+) instead of neutral M in the MS info panel
• calculate the displayed value by subtracting the electron mass from the exact mass
• add a focused test for the M to M+ conversion

Test plan

• open an MS spectrum and check that the info panel shows Exact mass (M+)
• verify the displayed value is slightly lower than the neutral exact mass
• run the InfoPanel test