Savings calculator: improve reached-goal behavior and goal timing

[jenbreese]

Summary

This PR improves the Savings calculator’s handling of reached goals, zero-interest calculations, compounding-only growth, partial periods, and year-by-year results.

https://stanfordits.atlassian.net/browse/IFDM-172
https://stanfordits.atlassian.net/browse/IFDM-347

Changes

• Show the “You’ve already reached this goal” message using the standard warning color.
• Apply warning styling to both the savings goal and current balance fields.
• Disable dependent fields and display - when the current balance meets or exceeds the goal.
• Allow Savings calculations at 0% interest and show the standard amber warning.
• Display an explanation beneath $0.00 when compounding alone will reach the goal.
• Stop the year-by-year table when the goal is reached.
• Display partial final years to one decimal, such as 0.8 or 4.5.
• Round Time to Goal up to the first contribution period that actually reaches the goal.
• Allow a smaller final contribution when the regular contribution would exceed the goal.
• Keep partial-period headline results and breakdown totals consistent.

Human QA scenarios

1. Goal already reached

On the Savings tab, enter:

• Savings goal: $1,000
• Current balance: $1,200

Expected:

• Both fields have amber warning styling.
• Both show the reached-goal warning.
• Time, interest, and compounding fields are disabled and display -.
• Results display -.

2. Zero-interest calculation

Enter:

• Goal: $12,000
• Current balance: $0
• Time: 1 year
• Interest: 0%
• Compounding: Monthly

Expected:

• The 0% message is amber.
• Calculation remains active.
• Save each month: $1,000.00
• Total deposited: $12,000.00
• Interest earned: $0.00

3. Compounding reaches the goal without contributions

Enter:

• Goal: $1,000
• Current balance: $990
• Time: 10 years
• Interest: 5%
• Compounding: Monthly

Expected:

• Save each month: $0.00
• Message says the goal will be reached in 3 months.
• The breakdown stops at the goal rather than continuing for 10 years.
• Final table row displays approximately year 0.2 and ends at $1,000.00.

4. Partial-year breakdown

Enter a Savings scenario lasting:

• 4 years
• 6 months

Expected:

• The table contains rows for years 1, 2, 3, 4, and 4.5.
• The final row ends at the savings goal.
• No rows appear after 4.5.

Also test a ten-month scenario and confirm the final year displays as 0.8.

5. Whole contribution-period timing

On Time to Goal, enter:

• Goal: $1,000
• Current balance: $0
• Contribution: $300
• Interest: 0%
• Compounding: Monthly

Expected:

• Time to goal: 0 years 4 months
• Total deposited: $1,000.00
• Interest earned: $0.00
• The final contribution is effectively $100, rather than another full $300.
• The breakdown ends at $1,000.00.

6. Partial compounding period

On Savings, enter:

• Goal: $1,000
• Current balance: $500
• Time: 6 months
• Interest: 10%
• Compounding: Annually

Expected:

• Required contribution: approximately $475.60
• Total deposited: approximately $975.60
• Interest earned: approximately $24.40
• The breakdown displays year 0.5.
• The table and headline results both end at $1,000.00.

Regression checks

• Change the goal or balance so the reached-goal warning clears; dependent fields should become editable again.
• Confirm Reset clears warnings, disabled states, inputs, results, and the breakdown.
• Smoke-test Monthly, Weekly, Quarterly, and Annual compounding.
• Check light and dark modes.
• Check keyboard navigation and narrow iframe widths.
• Confirm switching between all three calculator modes clears prior values and results.

Verification completed

• yarn lint completes with the existing isOverflow dependency warning.
• yarn build:local passes.
• Targeted calculation checks passed for zero interest, positive interest, exact-period boundaries, weekly contributions, partial periods, and normal 12-month savings.

Scope note

The unreproduced review findings numbered #1 and #3 were not changed in this PR.

[majimmy88]

LGTM