Add flatpak packaging target

electron-builder already bundles @malept/flatpak-bundler, so this just
adds the target + a finishArgs set matching the app's actual needs:
wayland/x11 rendering, PipeWire for WebRTC screen/mic capture (on top
of the base pulseaudio socket), camera device access, and the
freedesktop notifications + portal bus names (native/linux-portals
handles file chooser/background/global-shortcuts through the latter).
Bumped runtime/base to 23.08 from electron-builder's very old 20.08
default.

Building this target requires flatpak + flatpak-builder installed on
the build machine (Linux-only, can't run on macOS at all), plus the
org.freedesktop.Platform/Sdk and org.electronjs.Electron2.BaseApp
23.08 runtimes from Flathub.

fluxer_desktop/electron-builder.config.cjs

@@ -1256,11 +1256,38 @@ module.exports = {
 				target: 'tar.gz',
 				arch: targetArchs,
 			},
+			{
+				target: 'flatpak',
+				arch: targetArchs,
+			},
 		],
 		desktop: {
 			entry: linuxDesktopEntry,
 		},
 	},
+	flatpak: {
+		runtimeVersion: '23.08',
+		baseVersion: '23.08',
+		finishArgs: [
+			// Wayland/X11 rendering
+			'--socket=wayland',
+			'--socket=x11',
+			'--share=ipc',
+			// OpenGL
+			'--device=dri',
+			// Camera (native/mac-* equivalents on Linux go through /dev/video*)
+			'--device=all',
+			// Audio output + WebRTC screen/mic capture via PipeWire
+			'--socket=pulseaudio',
+			'--socket=pipewire',
+			'--filesystem=home',
+			'--share=network',
+			// System notifications
+			'--talk-name=org.freedesktop.Notifications',
+			// XDG portals (file chooser, background, global shortcuts -- see native/linux-portals)
+			'--talk-name=org.freedesktop.portal.Desktop',
+		],
+	},
 	deb: {
 		packageCategory: 'net',
 		desktop: {