fix: Replace ES6+ syntax with ES5-compatible JS for IE compatibility

[Copilot]

IE (8 through 11) throws parse errors on the upload plugin page due to extensive use of ES6+ syntax — let/const, arrow functions, template literals, class, async/await, for...of, etc.

UploadSection.html

• class ConcurrentUploadQueue / class UploadManager → prototype-based constructors
• async/await → Promise chains with .then(onFulfilled, onRejected) (avoids .finally() which IE also lacks)
• Arrow functions, template literals, for...of, Array.from(), .flat(), .includes() → ES5 equivalents
• Optional catch binding catch {} → catch(e) {}
• Added null guard on webkitGetAsEntry (WebKit-only, absent in IE)

HtmlTemplate.html

• All let/const, arrow functions, template literals → ES5
• Array.from(...).map(b => ...) → explicit for loop
• .includes() → arrayIncludes() helper using .indexOf()
• Multi-arg classList.add('a', 'b') → separate single-arg calls (IE doesn't support multiple tokens)
• Default parameters → explicit if (x === undefined) guards
• window.matchMedia → null-checked; .addEventListener → .addListener fallback for older IE
• requestAnimationFrame → msRequestAnimationFrame / setTimeout(cb, 16) fallback
• fetch calls → guarded with typeof fetch === 'function'
• e.key → e.keyCode / e.which fallback for IE keyboard events
• Added Element.prototype.closest polyfill (IE has no native implementation)

Note: drag-and-drop folder upload relies on webkitGetAsEntry() which is WebKit-only and will remain non-functional in IE regardless of JS syntax fixes. Basic file listing and XHR-based upload will work in IE 10+.