add : introduce playground mode with live analysis

[capimichi]

Summary

• add a dedicated Playground page and sidebar entry for free play with both colors
• show live Stockfish top lines, move classification highlights, move icons, and PGN copy/board controls
• add the required COEP/COOP headers for the new route while leaving local package changes out of the PR

[capimichi]

Just for context, this feature is useful for exploring positions freely without committing to a bot game, while still keeping live engine feedback and an easy way to export the resulting PGN.

[GuillaumeSD]

@capimichi You can already do all of this in the analysis page, what more does this bring ?

[capimichi]

@GuillaumeSD After reviewing it I think you are right, just 2 things maybe i can help implement just to make it more clear:

1. Before the "Load game" button, maybe we can add a "New game" or "New playground", this can be useful, for example, if i loaded a game, but i want to clean it up and restart from scratch.
2. When i make an analysis from scratch, maybe i need the PGN of that sample game I'm doing (for various reasons). Right now when I make the moves myself, the "Copy PGN" button is disabled. Maybe it can keep track of custom changes made from the user manually

What do you think about?