feat(frontend): always show the Solana balance changes, and say when they are unknown

[sbpublic]

Motivation

The balance-changes section rendered only when a simulation produced one ({#if nonNullish(preview)}). A request OISY could not simulate therefore showed no balance-changes section at all — and an absent section is indistinguishable from a transaction that moves nothing, which is the most dangerous thing this review can imply.

The codebase already holds this principle for the party lists: the partial-review notice exists because "an empty list reads as an answer rather than as a gap". Balance changes was the one place that still vanished silently, leaving the user to infer the difference from a notice further up the modal.

Stacked on #14086.

Changes

• Render the section whether or not there is an answer.
• With a simulation, unchanged — headed Simulated balance changes.
• Without one, headed Balance changes, since nothing was simulated and the old heading would be a claim of its own. It states that OISY cannot determine which assets the transaction moves or how much, as MessageBox level="error" rather than a warning: it describes a question the review could not answer at all, not something for the user to weigh.
• New label and message in all 14 shipped locales, generated types refreshed via npm run i18n.
• PRODUCT.md records that the section is always present and what each heading means.

Tests

• npx vitest run src/frontend/src/tests/sol — 78 files, 2134 tests
• npm run check / npm run check:tests — 0 errors
• npx eslint src/frontend/src/sol src/frontend/src/tests/sol
• 2 new cases: the section states the unknown answer when there is no simulation, and does not once a simulation answered

Not visually verified — no screenshot of the section in either theme.

🤖 Generated with Claude Code — Claude Opus 5