Display LCMS data

[Nicolass67]

Summary

Add support for viewing LC/MS (liquid-chromatography / mass-spectrometry) datasets in the
spectra editor, alongside the existing NMR / IR / MS / CV layouts. An LC/MS measurement is
not a single trace but a time-resolved family of spectra: a chromatogram over retention
time, the mass spectrum at each retention time, and (optionally) a UV/Vis detector trace.
The editor should let a user navigate that family — pick a retention time and see the
mass spectrum at that time — and persist their edits through ChemSpectra.

Background / motivation

Today LC/MS data can only be stored, not inspected or processed in the editor. Users need
to read the chromatogram, locate a peak in time, and inspect the mass spectrum at that
retention time the same way they work with any other spectrum (peaks, integrations,
threshold, save). Without this, LC/MS files are opaque in the ELN.

Desired behaviour

1. Layout. A dedicated LC/MS layout that renders the measurement as a multi-graph
   view rather than a single trace.
2. TIC (chromatogram). Show the total-ion-current chromatogram over retention time
   (x-axis in minutes). Support both positive and negative polarity TICs.
3. Retention-time selection → MS. When the user clicks or selects a retention time on
   the chromatogram, display the mass spectrum (m/z vs intensity) recorded at that time.
   Moving the selection updates the MS view.
4. UV/Vis detector. When the dataset includes a UV/Vis channel, display it and let the
   user choose the wavelength to view.
5. Editing parity. Within the LC/MS layout the standard tools work on the active
   trace: peak add/remove, integration, threshold, and decimal-precision control.
6. Input formats. Parse LC/MS data from JCAMP, including Agilent ChemStation exports
   (TIC, m/z pages, UV/Vis). Polarity (pos/neg) and detector type are taken from the data.
7. Persistence. Edits (peaks, integrations, selected wavelength/page) survive
   save/reload through ChemSpectra.

Acceptance criteria

• Opening an LC/MS dataset shows the chromatogram plus a linked MS view.
• Selecting a retention time updates the MS view to that time's spectrum.
• Positive and negative TICs are distinguished correctly (polarity comes from the data,
  not from trace ordering).
• UV/Vis channel is viewable with a wavelength selector when present.
• Peaks / integrations / threshold behave on LC/MS traces and round-trip through save.
• ChemStation JCAMP exports load without dropping pages.
• No regression to the NMR / IR / MS / CV layouts (peak-add, integration math, CV
  current-density all unchanged for non-LC/MS spectra).

Notes / references

• Original intent: Display LCMS data #231. Implementation PR: 231 display lcms data #232 (Closes #254).