web/api: jwt session token, clean up, move related modules to own dir

web/src/lib/state/session.ts

@@ -0,0 +1,6 @@
+import { writable } from "svelte/store";
+
+import type { Writable } from "svelte/store";
+import type { CobaltSession } from "$lib/types/api";
+
+export const cachedSession: Writable<CobaltSession | null> = writable();