feat(*): Add upload functionality

src/routes/upload/+page.svelte

@@ -1,12 +1,89 @@
 <script lang="ts">
-// Upload logic will go here
+import { onMount } from 'svelte';
+
+let file: File | null = null;
+let textInput = '';
+let submitting = false;
+let successMsg = '';
+let errorMsg = '';
+
+function handleFileChange(event: Event) {
+  const target = event.target as HTMLInputElement;
+  if (target.files && target.files.length > 0) {
+    file = target.files[0];
+    textInput = '';
+  } else {
+    file = null;
+  }
+  successMsg = '';
+  errorMsg = '';
+}
+
+function handleTextChange(event: Event) {
+  const target = event.target as HTMLTextAreaElement;
+  textInput = target.value;
+  if (textInput.length > 0) file = null;
+  successMsg = '';
+  errorMsg = '';
+}
+
+async function handleSubmit(event: Event) {
+  event.preventDefault();
+  submitting = true;
+  successMsg = '';
+  errorMsg = '';
+
+  // TODO - Actually do an upload, remove simulation of a successful upload
+  await new Promise((resolve) => setTimeout(resolve, 800));
+  if (file || textInput.trim().length > 0) {
+    successMsg = 'Submission received! AI assessment coming soon.';
+  } else {
+    errorMsg = 'Please provide a file or enter text.';
+  }
+  submitting = false;
+}
 </script>
 
 <section class="max-w-xl mx-auto py-12">
   <h1 class="text-2xl font-bold mb-4">Upload Student Submissions</h1>
   <p class="mb-6">Upload student assignments for instant AI-powered assessment and feedback.</p>
-  <form>
-    <!-- File or text input UI will go here -->
-    <button class="btn btn-primary mt-4" type="submit" disabled>Upload (Coming soon)</button>
+  <form on:submit|preventDefault={handleSubmit} class="space-y-6">
+    <div>
+      <label for="file" class="block font-semibold mb-1">Upload a file (PDF, DOCX, TXT):</label>
+      <input
+        id="file"
+        type="file"
+        accept=".pdf,.doc,.docx,.txt"
+        class="block w-full border rounded p-2"
+        on:change={handleFileChange}
+        disabled={submitting}
+      />
+    </div>
+    <div class="text-center text-gray-500">or</div>
+    <div>
+      <label for="textInput" class="block font-semibold mb-1">Paste assignment text:</label>
+      <textarea
+        id="textInput"
+        rows="6"
+        class="block w-full border rounded p-2"
+        bind:value={textInput}
+        on:input={handleTextChange}
+        disabled={submitting}
+        placeholder="Paste or type student answer here..."
+      ></textarea>
+    </div>
+    {#if errorMsg}
+      <div class="text-red-600 font-medium">{errorMsg}</div>
+    {/if}
+    {#if successMsg}
+      <div class="text-green-600 font-medium">{successMsg}</div>
+    {/if}
+    <button
+      class="btn btn-primary mt-4 w-full bg-blue-600 hover:bg-blue-700 text-white font-semibold py-2 px-4 rounded disabled:opacity-60"
+      type="submit"
+      disabled={submitting || (!file && textInput.trim().length === 0)}
+    >
+      {submitting ? 'Uploading...' : 'Upload'}
+    </button>
   </form>
 </section>