[nightshift] docs-backfill: add JSDoc documentation to all 21 JS modules

[nightshift-micr]

Automated by Nightshift v3 (GLM 5.1).

Task: docs-backfill (Documentation Backfiller)
Category: pr
Changes: Added JSDoc documentation to all 21 JavaScript modules in js/ directory.

Summary

Adds comprehensive JSDoc @fileoverview, @param, @returns, @typedef, and @exports annotations to every JS module:

• gallery-shared.js — 12 functions documented (sanitization, formatting, loading, rendering)
• layout.js — Desktop scaling system (DESIGN_WIDTH/HEIGHT, computeScale, syncLayout, toDesignPx)
• drag.js — Window dragging with mouse/touch support (makeDraggable)
• controls.js — Volume slider and animation toggle
• background.js — Color conversion utilities (hexToRgb, rgbToHsl)
• tabs.js — 98.css tab navigation with keyboard support
• gallery.js — Desktop gallery viewer with metadata
• warning.js — Warning popup and title-bar control interceptor
• tiles.js — Background tile switcher
• sounds.js — Windows 98 sound effects player
• notepad.js — Random notepad content with font/wrap controls
• clippy.js — Quote display system
• debug.js — Debug mode with resize grips
• mobile-*.js — Mobile gallery, viewer, tabs, sounds
• res-warning.js — Resolution compatibility warning
• logo.js — Random logo selector
• run.js — Console branding
• app.js — Main application bootstrap

21 files changed, +276 lines of documentation. No functional changes.

Merge if useful, close if not.