fix(console): render unknown utilisation as unknown

[albertovincenzi]

Problem

The API deliberately returns utilisation: null for scoped budgets because there is no single live counter. The shared Vue helper fell back to (value ?? 0) / ceiling, turning that explicit unknown into 0%. Budget History then displayed an empty green bar as if nothing had been spent.

Fix

• preserve an explicit null utilisation;
• return unknown when neither value nor used exists or the ceiling is unusable;
• allow BudgetBar to receive a null used value;
• render an explanatory unknown state instead of a zero-width green fill;
• stop Budget History from coercing null to zero.

Review notes for Alice

Known zero remains 0%; only missing/null data becomes —. Existing overview and shared-budget bars continue to pass numeric values. The helper change centralizes the semantic distinction already documented in the API code.

Verification

• npm run build
• direct Node assertions for explicit null, absent value, and a known 5/10 value