🎨 Palette: Add for attributes to form labels for accessibility

[longestmt]

💡 What

Added for attributes to <label> elements linking them to their corresponding input ids across modal forms in exercises.js, plans.js, and workout.js.

🎯 Why

Without explicit for attribute associations, screen readers cannot properly announce the input's purpose to users. Additionally, linking labels to inputs improves general usability by allowing users to click the label text to automatically focus the associated input field, increasing the clickable target area.

📸 Before/After

There are no visual changes, as the improvement purely modifies the underlying semantic HTML to enhance accessibility.

♿ Accessibility

Screen readers will now correctly associate all form labels with their respective inputs, selects, and textareas, greatly improving the experience for visually impaired users.

---

PR created automatically by Jules for task 14899080922754192206 started by @longestmt

[google-labs-jules]

👋 Jules, reporting for duty! I'm here to lend a hand with this pull request.

When you start a review, I'll add a 👀 emoji to each comment to let you know I've read it. I'll focus on feedback directed at me and will do my best to stay out of conversations between you and other bots or reviewers to keep the noise down.

I'll push a commit with your requested changes shortly after. Please note there might be a delay between these steps, but rest assured I'm on the job!

For more direct control, you can switch me to Reactive Mode. When this mode is on, I will only act on comments where you specifically mention me with @jules. You can find this option in the Pull Request section of your global Jules UI settings. You can always switch back!

New to Jules? Learn more at jules.google/docs.

---

For security, I will only act on instructions from the user who triggered this task.