feat(ux): Monospace/resizable editor and localStorage persistence per challenge

[ArchILLtect]

Problem / context

Improve code entry experience in the solve view.

Proposed solution

Use monospace styling, make textarea resizable, and persist user code in localStorage keyed by challengeId

In scope / Out of scope

In scope

• Front-end only

Out of scope

• Third-party editor integrations

Acceptance criteria

• Monospace/resizable editor; code persists per challengeId on reload

Area

area:ui/ux

Dependencies / related issues

No response

Checklist

• I’ve checked existing issues for duplicates.
• I can help implement this and open a PR.