fix: surface unknown pricing instead of misleading $0.0000

[alexdim]

What

• isFreeModel: true only when inputPrice === 0 && outputPrice === 0
• getCostLabel: n/a | FREE | $X.XXXX (paid with no usage → $0.0000)
• Regression: paid model + totalCost === 0 → $0.0000, not FREE
• Cost calc returns undefined when pricing unknown (not 0)

Why

hasPricing() means prices exist, not free. Paid models started at totalCost === 0 and were labeled FREE until the first billed response.

Notes / Caveats

• Addresses CHANGES_REQUESTED on this PR
• Pre-existing biome Thumbnails indent in TaskHeader.tsx (also on master) left alone
• Pre-existing root tsc error in src/shared/storage/types.ts on master (delta 0)

Verification

• tsc delta vs master: 0 new errors (1 pre-existing on both)
• biome: no new errors on touched files
• mocha src/utils/cost.test.ts: 21 passing
• vitest getCostLabel.test.ts: 5 passing

User test

1. Paid model (e.g. Claude) before first response → $0.0000, not FREE
2. Explicit free model (inputPrice/outputPrice 0) → FREE
3. Unknown-pricing custom model → n/a
4. After billed usage on paid model → $X.XXXX

Alexandros Salapatas

[dirac-run]

hasPricing() only means that at least one price is known; it does not mean the model is free. Since totalCost starts at zero, this labels ordinary paid models as FREE until their first billed response. Please render a neutral $0.0000 state for known nonzero pricing and reserve FREE for explicitly zero-priced models. A regression test for a paid model with totalCost === 0 would cover this.