fix: AMS slot labels always show 'A' when selecting a non-first AMS unit

[BenJule]

Summary

• In the Add Filament → Read from AMS dialog, the slot labels (A1–A4) were hardcoded and never updated when the user switched to a second or third AMS unit.
• With multi-AMS setups, selecting AMS B would still display slot labels A1, A2, A3, A4 — matching none of the identifiers shown everywhere else in the UI (filament grouping, bound_ams_id field, etc.).
• Fix: derive the unit letter from currentUnit.ams_id ('0' = A, '1' = B, …) and construct slotLabels dynamically so they always reflect the currently selected AMS unit.

Change

One-line change in AddEditDialog.tsx:

// Before
const slotLabels = ['A1', 'A2', 'A3', 'A4'];

// After
const unitLetter = currentUnit ? String.fromCharCode('A'.charCodeAt(0) + parseInt(currentUnit.ams_id, 10)) : 'A';
const slotLabels = [1, 2, 3, 4].map((n) => `${unitLetter}${n}`);

Test plan

• Single AMS: slot labels still show A1–A4
• Two AMS units connected: switching to unit 2 shows B1–B4; switching back shows A1–A4
• Three+ AMS units: C1–C4 for unit 3, etc.

Fixes #10512

[BenJule]

Apologies — this PR was closed by mistake on 2026-06-07, and that was an error on my side, not a deliberate decision to withdraw the change.

It happened as an unintended side effect of a branch cleanup in my fork: deleting the head branch automatically closed this PR. The contribution still stands. I have restored the branch and reopened the PR. Sorry for the noise and any confusion this caused.