WebUI: Change ServerPathSeparator to be a getter function

[vafada]

This PR #24678 is implemented wrong

1. the ServerPathSeparator for windows will never be \

When filesystem.js is included the IIFE (Immediately Invoked Function Expression) is immediately called but the call to the buildInfo api might not resolve yet:

qBittorrent/src/webui/www/private/scripts/cache.js

Lines 53 to 74 in 52d3a96

	class BuildInfoCache {
	#m_store = {};
	init() {
	return fetch("api/v2/app/buildInfo", {
	method: "GET",
	cache: "no-store"
	})
	.then(async (response) => {
	if (!response.ok)
	return;
	const responseJSON = await response.json();
	deepFreeze(responseJSON);
	this.#m_store = responseJSON;
	});
	}
	get() {
	return this.#m_store;
	}
	}

so window.qBittorrent?.Cache.buildInfo.get().platform will be undefined and ServerPathSeparator will be stuck at /

To fix this, I replaced the constant to be a getter function

2. filesystem.js now depends on window.qBittorrent.Cache. But there are several html files that includes filesystem.js but never includes cache.js. This broke a couple of features in the WebUI with a javascript error.

• Add Torrent Dialog is broken
  • Add a new torrent via Add Torrent File... and Add Torrent Link...
  • in the RSS tab, when double-clicking to download
• In the Content tab, Rename file link

Again, the IIFE (Immediately Invoked Function Expression) in filesystem.js is invoked but window.qBittorrent?.Cache is undefined.

The optional chaining operator must be window.qBittorrent.Cache?.buildInfo (after the Cache object), though changing it to a getter function resolves this.

I still added <script defer src="scripts/cache.js?v=${CACHEID}"></script> to all files that includes filesystem.js since filesystem.js now depends on window.qBittorrent.Cache

[vafada]

Easy to test this on non-windows machine, just hardcode platformName to windows:

qBittorrent/src/webui/api/appcontroller.cpp

Lines 100 to 109 in 52d3a96

	const QString platformName =
	#ifdef Q_OS_LINUX
	u"linux"_s;
	#elif defined(Q_OS_MACOS)
	u"macos"_s;
	#elif defined(Q_OS_WIN)
	u"windows"_s;
	#else
	u"unknown"_s;
	#endif

Try the new Copy path feature in WebUI and check what the path separator is.