Optionally include bar chart renderings for klog report

[jotaen]

Related #336.

This PR adds a --chart (-c) flag to the klog report command, which includes bar chart diagrams in the output like so:

$ klog report --chart worktimes.klg

                       Total                                  
2025 Jan    Tue 14.       6h  ▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇        
            Thu 16.    7h30m  ▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇  
            Fri 17.    6h45m  ▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇     
            Sat 18.       8h  ▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇
            Mon 20.    3h20m  ▇▇▇▇▇▇▇▇▇▇▇▇▇▇                  
            Wed 22.    7h45m  ▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇ 
            Sat 25.    7h15m  ▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇   
                    ========                                  
                      46h35m                                  

The default resolution of the bars depends on the aggregation mode. (Remember: aggregation defaults to daily, unless specified via --aggregate/-a). The default resolution aims to make for a good balance between granularity and row width for typical usage scenarios (e.g., tracking work times). The resolution can be overridden with the --chart-res flag; it needs to be a positive integer, denoting the number of minutes per rendered block.

One implementation note for reference: I originally tried to use Unicode’s “eighths” type block characters to make the bars more precise. This didn’t work out so well, however, because on some systems, the full block and the “eighths” blocks render at a slightly different height, which looks weird.