fix(single-lease): use stable decimals for short size in header

[metodi96]

Summary

Fix the single-lease header's size readout for short positions. On a short, `lease.amount` is denominated in the stable the user holds, not in the LPN. The header was applying LPN decimals to the stable amount, producing values 100× off — a BTC short showing $0.69 instead of $69.98 while the Summary widget directly below showed the correct figure.

Changes

• `SingleLeaseHeader.vue` — replace `lpn.decimal_digits` with `asset.decimal_digits` (decimals of `lease.amount.ticker`) in the short branch of the `stable` computed. Drop the now-unused `getLpnByProtocol` lookup.

Verification

• Reproduced the bug on a BTC short: header showed $0.69, Summary showed $69.98. After the fix, header matched Summary ($69.98).
• Ran lint, vitest, and the worker build via the pre-commit hook — all clean.
• Cross-checked against `PositionSummaryWidget.vue:326` which already used `asset.decimal_digits` for the same lease, confirming the fix aligns the two surfaces on a single source of truth.