fix: add keyboard accessibility to Heatmap cells (JhaSourav07#2216)

## Description
Closes JhaSourav07#354

Adds proper ARIA grid semantics to the Heatmap component for full
keyboard accessibility.

**Changes made:**
- Added `role="grid"` to outer flex container
- Added `role="row"` to each week column
- Added `role="gridcell"` to each day cell

**Already present (no changes needed):**
- `tabIndex={0}` — cells are keyboard focusable
- `onFocus` → show tooltip
- `onBlur` → hide tooltip
- `aria-label` with contribution count and date
- `focus:ring-2` for visible focus indicator

**Why role="grid" pattern?**
`role="gridcell"` alone is insufficient — ARIA spec requires
gridcell to be inside a row, and row inside a grid.
This makes the heatmap fully accessible to screen readers.

## Pillar
♿ Accessibility

## Checklist
- [x] My code follows the project's coding standards
- [x] I have run `npm run format` and `npm run lint`
- [x] CI Pipeline passing ✅
- [x] No regressions in existing functionality
- [x] Closes JhaSourav07#354

Author: JhaSourav07

components/dashboard/Heatmap.tsx

@@ -128,15 +128,16 @@ export default function Heatmap({
                 height: (7 * (CELL + GAP) - GAP) * scale,
               }}
             >
-              <div className="flex" style={{ gap: GAP }}>
+              <div className="flex" role="grid" style={{ gap: GAP }}>
                 {weeks.map((week, wIndex) => (
-                  <div key={wIndex} className="flex flex-col" style={{ gap: GAP }}>
+                  <div key={wIndex} className="flex flex-col" role="row" style={{ gap: GAP }}>
                     {week.map((day, dIndex) => {
                       const originalIndex = wIndex * 7 + dIndex;
 
                       return (
                         <div
                           key={day.date}
+                          role="gridcell"
                           aria-label={`${getContributionLabel(
                             day.count
                           )} on ${formatTooltipDate(day.date)}`}