fix: month view selection

[vilnytskyi]

At the moment month selector is broken. The cause of issue is wrong computation of selected item's index based on current matrix row/column. This pull request fixes mentioned computation.

[codecov-io]

Codecov Report

Merging #57 into dev will not change coverage by %. The diff coverage is 100.00%.

@@            Coverage Diff            @@
##               dev       #57   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           26        26           
  Lines          640       640           
  Branches        71        71           
=========================================
  Hits           640       640           

Impacted Files	Coverage Δ
src/components/TableMatrixView.tsx	100.00% <100.00%> (ø)

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Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 100.00%. Comparing base (042930b) to head (b0fac00). Report is 8 commits behind head on dev.

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Additional details and impacted files

```diff @@ Coverage Diff @@ ## dev #57 +/- ## ========================================= Coverage 100.00% 100.00% ========================================= Files 26 26 Lines 640 640 Branches 71 71 ========================================= Hits 640 640 ```

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