Refactor (3/N): split js/app.js into per-feature files

[dnkats]

Third step of splitting the large index.html. This carves the single js/app.js (4,121 lines) into 14 cohesive feature files.

Safety: provably behavior-preserving

The files are loaded as classic scripts in the same source order as the original code, so:

• they share one global scope → the 49 inline onclick= handlers and all cross-file function/variable references keep working;
• top-level statements (the var/let/const globals, the Info object, and the $(document).ready/addEventListener registrations) execute in the exact same order.

I verified mechanically that concatenating the 14 files in load order is byte-for-byte identical to the previous js/app.js — i.e. this is the same code, just partitioned. Each file also parses independently (node --check), confirming no split lands mid-statement.

The files (load order)

File	Contents
jsmol-setup.js	globals + Info object (JSmol applet config & callbacks)
image-export.js	PNG/JPEG export
molecule-data.js	atom-name preservation, XYZ loading, $(document).ready startup
viewer-controls.js	spin / display mode / zoom
search.js	file & sample load, PubChem/database search
orbitals.js	MO list + navigation
xyz-editor.js	XYZ editor panel
selection.js	shared atom-selection state
ui.js	draggable panels, wheel zoom, resize observer
jsme-2d.js	2D/3D (JSME) integration
elemco.js	ElemCo.jl input generation + Julia runner
xtb.js	xtb (g-xTB) energy/optimization
app-init.js	panel + application initialization
preferences.js	user preferences system

js/**/* is already in the build.files allowlist (from PR #23), so packaging is covered.

Notes

• Indentation is left at the original 8 spaces so this remains a pure move (no whitespace noise). An optional whitespace-only re-indent PR can follow if desired.
• A natural cleanup for a later PR: renderer.js still redefines a few functions (copyElemCoInput, showElemCoPanel, …) that the app overrides — worth consolidating.

Testing

• Concatenation == original app.js (verified).
• node --check passes for all 14 files.
• Recommend an npm start smoke test across the panels (load molecule, XYZ editor, selection both ways, orbitals, ElemCo, xtb).

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