refactored timetracking to only use client-supplied times

frontend/src/Components/TimeStampInput.svelte

@@ -0,0 +1,16 @@
+<script>
+    import { display, to_secs } from '../time.svelte.js';
+    let { timestamp = $bindable() } = $props();
+
+    let datetime = $state(null);
+
+    $effect(() => {
+        datetime = display(timestamp);
+    });
+
+    $effect(() => {
+         timestamp = to_secs(datetime);
+    });
+</script>
+
+<input type="datetime-local" bind:value={datetime} />