Repurpose `src/utils` into composables

[n-lark]

Repurpose src/utils into composables

Summary

Migrate the helpers in frontend/src/utils/ into frontend/src/composables/. Composables are the idiomatic Vue 3 way to share stateful and reactive logic; the loose utils grab-bag predates that pattern.

Scope

• Audit src/utils and categorise each helper:
  • Stateful / reactive / lifecycle-bound → convert to a useX composable.
  • Pure, stateless functions → keep as plain utils (a composable adds nothing).
• Update all imports.
• Move/extend tests alongside the new composables.

Acceptance criteria

• Reactive/stateful helpers live in src/composables as useX.
• Pure functions either retained as utils or co-located with clear rationale.
• All imports updated; build and tests pass.