Add helper for displaying total portfolio value across all held keys

[sochima2]

Summary

Description
Adds a new helper calculatePortfolioValue with formatPortfolioValueDisplay and getPortfolioValueHelperText under src/utils/portfolioValue.utils.ts that aggregates held key positions and returns ready | loading | unavailable status along with counts and total stroops.
Integrates the helper into the marketplace (src/pages/LandingPage.tsx) via heldKeyPositions/portfolioValue memos and renders a new "Holdings overview" card that shows the formatted total, per-position price states, and an accessible loading/unavailable indicator.
Handles edge cases: zero holdings resolve to a zero total, any isPriceLoading yields a loading state, and missing or stale prices result in an unavailable state so partial totals are never shown as final.
Adds unit tests covering summed totals, zero holdings, loading state, missing price data, and stale price data at src/utils/tests/portfolioValue.utils.test.ts.

Testing

Testing
Ran unit tests with pnpm exec vitest run src/utils/tests/portfolioValue.utils.test.ts and all new tests passed (5 passed).
Performed type checking with pnpm exec tsc -b which completed successfully.
Ran lint via pnpm lint which completed with no new errors.
Built the app with pnpm build; the production build succeeded (build warnings about chunk size / injected comments are unchanged existing warnings).

• pnpm lint
• pnpm build

Checklist

• Linked issue or backlog item
• Scope is limited to the stated change
• Updated docs if behavior or setup changed
• Added screenshots for UI changes when relevant

close #376

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