fix(frontend): parse limit-order units safely

[cursor]

Motivation

Limit-order submission converts the reviewed human base amount and price into candid bigint units. The merged helpers multiplied JavaScript number values by 10 ** decimals and passed .toFixed(0) into BigInt. For 18-decimal tokens, valid values such as 1000 scale to 1e+21, which BigInt rejects, and high-precision decimal values can also be rounded to a different smallest-unit amount than the UI reviewed.

Awareness checked before and after the fix: recent merged limit-order PRs #13253, #13254, #13255; open stacked limit-order PRs #13256-#13268 and staging branch #13220; related OISY TRADE stale-principal PR #13246; PR bodies, reviews/comments, commit messages, docs/spec docs/ai/spec-driven-development/specs/2026-06-04-feat-limit-orders.md, open/recent issues, and PR/issue searches for toQuantity, toPriceUnits, limit-order precision, and OISY TRADE conversion. No active or merged work was found that fixes this conversion trigger; the open stack makes the helpers reachable but does not cover this bug.

Changes

• Convert limit-order quantity and price units through the existing decimal token parser instead of floating-point smallest-unit multiplication.
• Round only to the token's declared decimals before parsing, avoiding binary artifacts while preserving the reviewed decimal value.
• Add regression coverage for large 18-decimal values that previously produced exponential BigInt input and for decimal precision preservation.

Tests

• PATH="/home/ubuntu/.nvm/versions/node/v24.18.0/bin:$PATH" npm run prepare
• PATH="/home/ubuntu/.nvm/versions/node/v24.18.0/bin:$PATH" npm run test -- --run src/frontend/src/tests/lib/utils/oisy-trade.utils.spec.ts
• PATH="/home/ubuntu/.nvm/versions/node/v24.18.0/bin:$PATH" npm run format
• PATH="/home/ubuntu/.nvm/versions/node/v24.18.0/bin:$PATH" npm run lint -- --max-warnings 0
• PATH="/home/ubuntu/.nvm/versions/node/v24.18.0/bin:$PATH" npm run check
• PATH="/home/ubuntu/.nvm/versions/node/v24.18.0/bin:$PATH" npm run test (915 files passed, 1 skipped; 15685 tests passed, 10 skipped, 1 todo)
• PATH="/home/ubuntu/.nvm/versions/node/v24.18.0/bin:$PATH" npm run lint:lockfile