🎨 Palette: Add progressive character limit warnings

[Sczitzo]

💡 What: Added progressive visual feedback to the character counter on the Alternative Thought input in ResponseForm. The counter turns orange at 80% capacity (240 characters) and red at 100% (300 characters). Also added screen reader-only status messages and removed redundant aria-describedby links.
🎯 Why: Users were not given adequate warning before hitting the hard character limit. Screen reader users were previously subjected to noisy aria-live updates on every keystroke, which is now replaced with static status announcements at critical thresholds.
📸 Before/After: Visual counter stays gray until 240 chars, turns orange medium-weight, then red bold at 300 chars.
♿ Accessibility: Removed duplicate announcement of "(Optional)" by dropping redundant aria-describedby. Implemented non-intrusive role="status" screen reader warnings for approaching and hitting character limits instead of wrapping the dynamic counter in an aria-live region.

---

PR created automatically by Jules for task 8348785636426729240 started by @Sczitzo

[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.