fix: investment form leading zeros, nav prop mismatch, decimal rounding + add return projection

[SarahDoma]

Summary

Four small, focused fixes/enhancements to the investment (deposit) flow:

• Leading zeros in the amount input — sanitizeAmount now strips leading zeros from the whole-number part as the user types, so 007 becomes 7 instead of displaying an unprofessional zero-padded amount. 0 and 0.5 still work correctly while typing.
• onNav ignoring its argument — Landing's onNav: (screen: Screen) => void prop was only ever called with 'explore'. Renamed to onExplore: () => void in Landing.tsx, src/app/page.tsx, and src/app/verify/page.tsx so the prop's type now matches its actual, single behaviour.
• Inconsistent decimal rounding — added roundToDecimals / roundToCents / formatDecimal to src/lib/format.ts (decimal-precision rounding, avoiding binary floating-point artifacts like (1.005).toFixed(2) === '1.00'). Deposit.tsx and Withdraw.tsx now round the entered amount once to cents and reuse that single rounded value everywhere it's displayed (pay amount, fee, net proceeds, review row), instead of each .toFixed() call re-rounding the raw float independently.
• Investment projection — added projectedReturn(amount, annualYieldPct, years) to src/lib/bondUtils.ts and a real-time "Projected return in 1 year" line next to the deposit amount input, computed from the pool's bond yield (HB_DATA.pool.projectedRate). New Deposit.projection key added to both messages/en.json and messages/fr.json.

Linked issues

Closes #370
Closes #373
Closes #369
Closes #367

Type of change

• Bug fix
• Feature

Testing

• Added unit tests for the leading-zero stripping behaviour in AmountInput.test.tsx.
• bun run build compiles successfully (Turbopack); the TypeScript check step surfaces pre-existing type errors unrelated to this change (present on main prior to this branch, verified by diffing typecheck output against main).
• bun run lint — no new lint errors introduced (verified by diffing against main).
• bun run format:check — all files touched by this PR are formatted; remaining warnings are in files not touched by this PR.
• bun run test — all new/existing tests around the changed code pass; the 3 pre-existing failing test files (useSessionTimeout.test.ts, AdminConsole.test.tsx) fail identically on main and are unrelated to this change.

Checklist

• Tied to an accepted issue (Closes #…)
• Follows the design system — token CSS vars, no hardcoded colours
• User-facing strings added to both messages/en.json and messages/fr.json
• No secrets committed

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[sshdopey]

This is a lovely, well-scoped PR that tackles four distinct issues with care. The leading-zero fix, the prop rename, the consistent rounding helpers, and the new projection feature all directly address the linked issues and include tests. The code is clean and the changes are focused. Great work, Sarah! 🎉 Keep it up!