Use MNE as source of truth for canonical montages

The previous hand-rolled spherical formulas in montages.js were off by up
to 76° for Fp1/Fp2 and 43° for T7/T8 compared to MNE's canonical templates.
The bug: sph(az, el) treated 'el' as elevation from equator, but the
formula's call sites passed values intending polar-from-vertex. Symmetric
54° offset error throughout.

Replacement: scripts/build-canonical-montages.py pulls MNE's 12 canonical
templates (standard_1020, standard_1005 filtered for 10-10 / full 10-05,
biosemi 16/32/64/128/256, GSN-HydroCel 128/256, mgh60, easycap-M1) and
emits a frozen JSON. montages.js becomes a 16-line loader that just
inlines the data.

For BioSemi 256 specifically, A1 is now at (0, 0, 0.095m) = vertex, which
matches BioSemi's published layout and FieldTrip's biosemi256.lay
reference. Previously A1 rendered wherever the dataset-specific TSV
placed it; now the built-in 'BioSemi 256' montage ships MNE's canonical
positions.

Also added biosemi256, egi256, mgh60, and easycap-M1 to the montage
picker (previously only 8 options; now 12).

Regenerate: python3 scripts/build-canonical-montages.py

Author: bruAristimunha

index.html

@@ -8,7 +8,7 @@
 <link rel="preconnect" href="https://fonts.gstatic.com" crossorigin />
 <link rel="stylesheet" href="https://fonts.googleapis.com/css2?family=IBM+Plex+Mono:wght@400;500;600&family=IBM+Plex+Sans:wght@400;500;600&display=swap" />
 <link rel="stylesheet" href="styles.css?v=4" />
-<script src="montages.js?v=4"></script>
+<script src="montages.js?v=5"></script>
 <script src="bids-loader.js?v=2"></script>
 <script src="topo2d.js?v=6"></script>
 </head>
@@ -211,7 +211,12 @@ <h3 class="rail-title"><span>Quick actions</span></h3>
 
 /* Montage list */
 const montageList = document.getElementById('montage-list');
-const MONTAGE_ORDER = ['10-20', '10-10', '10-05', 'biosemi16', 'biosemi32', 'biosemi64', 'biosemi128', 'egi128'];
+const MONTAGE_ORDER = [
+  '10-20', '10-10', '10-05',
+  'biosemi16', 'biosemi32', 'biosemi64', 'biosemi128', 'biosemi256',
+  'egi128', 'egi256',
+  'mgh60', 'easycap-M1',
+];
 function renderMontageButton(btn, label, count) {
   btn.textContent = '';
   const name = document.createElement('span');