Add a manual light/dark mode switcher

Despite the simplicity of the commit title, this was a pretty big
change. The styling used to just go off of the system's color scheme,
but that can't be overridden. Instead, I have made a variable that
determines whether dark theme is active and made a small panel with some
buttons to change the theme. I had to change a lot of code to achieve
this and lost a lot of hair (I metaphorically pulled it out) from
writing this code.

I also changed things from legacy mode to rune mode (Svelte 4 to 5)
while I was at it, that wasn't too big.

src/script/theme.ts

@@ -0,0 +1,15 @@
+/*/
+ * I know that having a source file this small is a bit silly,
+ * but I think it's probably a good way to do this. Since multiple
+ * components need the theme enum, why not define it in one place.
+ * I also think it's funny for a source file to have more comments
+ * than code. This will probably be changed in a future refactor.
+ *
+ * Also JavaScript enums suck (hot take)
+/*/
+
+export var themes = {
+	LIGHT: "light",
+	DARK: "dark",
+	AUTO: "auto",
+};