fix(frontend): stabilize grouped dashboard account metrics

[AtomicTrxn]

Description

Root cause:

• grouped child account rows on the dashboard hid the secondary metric line whenever gain data was zero or unavailable
• that made some expanded rows render with two lines and others with one line, which looked like rows were switching between balance and gain displays
• the primary metric was already correct; the bug was inconsistent secondary-metric rendering

Fix summary:

• keep the primary line as account value for all rows
• for nested expanded group rows, always render a secondary line
• show gain amount/percent when gain data exists
• show neutral zero-gain output when gain is zero
• show - when valuation/gain data is unavailable
• keep group headers and standalone account cards unchanged

Validation:

• pnpm --filter frontend test -- accounts-summary
• pnpm --filter frontend type-check
• pnpm --filter frontend lint
• pnpm exec prettier --check apps/frontend/src/pages/dashboard/accounts-summary.tsx apps/frontend/src/pages/dashboard/accounts-summary.test.tsx

Checklist

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CLA.

[afadil]

changed to logic in this pr #759