feat: after-tax projections & UI improvements

[halfguru]

This PR implements after-tax projections, UI refinements, and consolidated tab navigation:

Key Highlights:

1. After-Tax Projections:

   • Introduced persistent retirementTaxRate assumption in AssumptionsContext.
   • Updated calculation engine (calculations.ts / useRetirementGoal.ts) to deduct estimated tax rate from the RRSP balance, yielding After-Tax Net Worth and After-Tax Progress.
   • Corrected the solver to accurately calculate the additional savings rate needed to cover tax liabilities.

2. Tab Consolidation:

   • Merged the Income tab components directly into the Overview tab layout.
   • Placed the interactive retirement income donut breakdown side-by-side with the current portfolio card.
   • Moved detailed pension/CPP descriptions to the bottom footer of the Overview tab.

3. UI Refinements:

   • Redesigned the SummaryCard into a clean, grid-based dashboard KPI panel.
   • Standardized Header action buttons to uniform heights and center alignment.
   • Left-aligned SVG donut charts and prevented vertical stretching.