Add multiple editor tabs

[makermelissa-ai-assistant]

Summary

• add an editor tab strip so multiple files or scratch buffers can stay open at once
• preserve a separate CodeMirror state, active filename, and saved-change marker for each tab
• open files into new tabs, switch to already-open paths, and prompt before closing dirty tabs
• keep the existing serial/connection workflow shared across tabs

Refs #352

Testing

• npm run build
• git diff --check
• browser smoke test at http://127.0.0.1:4222/: verified initial tab render, dirty tab marker after editing, no console errors, and no horizontal overflow at 390px mobile width

[makermelissa]

I'm going to continue testing. I've found a number of small issue that have been resolved, but I'm not confident this is the final version until I test more thoroughly. So far it's looking great.

[makermelissa]

Ok, after some fairly thorough testing, I think this one is good to go.