🎨 Palette: improve session creation button accessibility and status feedback

[ightevenmckane187]

💡 What: Added aria-label="Create Session" to #create-session-btn and an adjacent aria-live="polite" status element (#session-create-status) in src/server.ts.
🎯 Why: Assistive technologies and visual users require explicit status feedback on asynchronous session creation results rather than relying solely on temporary button text replacement.
♿ Accessibility: Uses aria-live="polite" for non-disruptive screen reader announcements and provides explicit aria-label attribute on the session creation button.

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PR created automatically by Jules for task 2064118582648211752 started by @ightevenmckane187

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