Deck Editor Polish and Deck Export Fix

Staging release — try it at staging.arcanetable.app.

Fixes

• Deck export — exported .txt files no longer include cards that are currently in play on the battlefield.
• Disabled states — "Start Playtest" and "Edit Deck" are now correctly disabled when no deck is selected. Previously the wrong buttons were being gated.
• Tags field — pressing Enter in the deck tags field no longer submits the form.
• Deck mutation — the deck is now cloned before being passed into the editor, so edits can't accidentally affect your saved deck data.
• Add card button — the click handler is now on the button element rather than the icon inside it, fixing interaction issues in some browsers.
• Starting life total — clearing the starting life field no longer produces NaN; it falls back to 0.
• Deck picker close button — removed the close button from the session start dialog to prevent accidentally dismissing it.

Improvements

• The delete deck confirmation dialog now includes a copy button next to the deck name, so you don't have to type it manually.