Fix <textarea> Resizing Issue

Updated the textarea element within the settings page with two changes:
- The textarea now cannot be resized, and is always set to fill 100% of its container.
- The placeholder is now "Enter your own prompt for AI reflections" instead of "Paste an OpenAI API key to enable AI reflections". You can't see this because the textarea is currently readonly, but I assume this will change in the near future.

src/renderer/pages/Pile/Settings/Settings.module.scss

@@ -185,6 +185,8 @@ input {
     font-size: 14px;
     line-height: 1.35;
     min-height: 150px;
+    width: 100%; /* Ensure the width is 100%, no less, no more */
+    resize: none; /* Prevent resizing */
     color: var(--primary);
     background: var(--bg-tertiary);
     border: none;

src/renderer/pages/Pile/Settings/index.jsx

@@ -96,7 +96,7 @@ export default function Settings() {
             </label>
             <textarea
               className={styles.Textarea}
-              placeholder="Paste an OpenAI API key to enable AI reflections"
+              placeholder="Enter your own prompt for AI reflections"
               value={prompt}
               readOnly
             />