feat: add cobalt editor theme

[bajrangCoder]

Closes: #2476

[greptile-apps]

Greptile Summary

This PR adds a Cobalt editor theme. The main changes are:

• New Cobalt theme colors and CodeMirror styling.
• New syntax highlighting rules for the theme.
• Registration of Cobalt in the theme list.

Confidence Score: 5/5

This looks safe to merge.

• No blocking issues found in the changed code.

Important Files Changed

Filename	Overview
src/cm/themes/cobalt.js	Adds the Cobalt theme config, editor styling, highlighting rules, and theme factory.
src/cm/themes/index.js	Registers the Cobalt theme with the existing theme registry.

_{Reviews (2): Last reviewed commit: "remove ext" | Re-trigger Greptile}