Add visual background color indicators for speech item states

[paulpv]

Add visual feedback showing which items are queued, currently speaking, or already spoken.
This helps users/dev understand what's happening and troubleshoot issues.

Color States:

1. Enqueued (not yet spoken): Light blue background (rgba(33, 150, 243, 0.15))
2. Currently speaking: Translucent yellow background (rgba(255, 255, 0, 0.25)) - already implemented
3. Already spoken: Light gray background (rgba(0, 0, 0, 0.05))

Features:

• Only visible when Debug Mode is enabled (see Issue Implement generic spoken text filter, especially for markdown (ex: "===...", bullets, numbered lists, etc) #10
• Add/remove CSS classes or inline styles as speech state changes
• Preserve original background color in data attribute for restoration
• Clear indicators when navigating away or reloading

Implementation:

• Hook into addSpokenItem() for enqueued state
• Hook into speech start event for speaking state
• Hook into speech end event for spoken state
• Use data attributes to track state: data-tts-state="enqueued|speaking|spoken"