Restrict diff line selection to only existing sides — radicle-desktop

Instead of allowing users to click on left and right side of diff lines, we only should allow people to select on the available side.

If we have README.md:L1 but no R1 we shouldn’t allow users to write a comment on README.md:R1 since this will fail.

               isSelected(filePath(file, "left"), hunkIdx, lineIdx)}>
             <div
               class="left"
               class:selection-disabled={!codeComments || thread}
               class:selection-disabled={!codeComments ||
                 thread ||
                 lineNumber(line, "left") === undefined}
               class:marker={selection?.start.side === "left" &&
                 selection.start.lineNumber === lineNumber(line, "left")}
               onpointerdown={e => {

             <div
               class="right"
               class:selection-disabled={!codeComments || thread}
               class:selection-disabled={!codeComments ||
                 thread ||
                 lineNumber(line, "right") === undefined}
               class:marker={selection?.start.side === "right" &&
                 selection.start.lineNumber === lineNumber(line, "right")}
               onpointerdown={e => {